So, finaly, I directly tried to reinstall OpenWRT following this process (method 2).
After creating a TFTPD server on my laptop with tftpd64, I executed the following commands (full log below - I don't forgate to allow TFTP on my Windows firewall, the second time 
) :
GS750E# rtk network on
GS750E# setenv serverip 192.168.1.234;
GS750E# tftpboot 0x8f000000 openwrt-23.05.3-realtek-rtl839x-netgear_gs750e-initramfs-kernel.bin;
Using eth#0 device
TFTP from server 192.168.1.234; our IP address is 192.168.1.1
Filename 'openwrt-23.05.3-realtek-rtl839x-netgear_gs750e-initramfs-kernel.bin'.
Load address: 0x8f000000
Loading: #################################################################
#################################################################
#################################################################
#################################################################
#################################################################
#################################################################
#################################################################
######
done
Bytes transferred = 6762745 (6730f9 hex)
GS750E# bootm;
Then, I have OpenWRT up and running but I was unable to connect on the web interface for the regular firmware upgrade. But configuration seems to be ok.
root@OpenWrt:/# uci export network;
package network
config interface 'loopback'
option device 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'fdfb:c33b:0902::/48'
config device 'switch'
option name 'switch'
option type 'bridge'
option macaddr '38:94:ed:7e:19:60'
config bridge-vlan 'lan_vlan'
option device 'switch'
option vlan '1'
option ports 'lan0 lan1 lan2 lan3 lan4 lan5 lan6 lan7 lan8 lan9 lan11 lan12 lan13 lan14 lan15 lan16 lan17 lan18 lan19 lan20 lan21 lan22 lan23 lan24 lan25 lan26 lan27 lan28 lan29 lan30 lan31 lan32 lan33 lan34 lan35 lan36 lan37 lan38 lan39 lan40 lan41 lan42 lan43 lan44 lan45 lan46 lan47 lan48 lan49 lan50'
config device
option name 'switch.1'
option macaddr '38:94:ed:7e:19:60'
config interface 'lan'
option device 'switch.1'
option proto 'static'
option ipaddr '192.168.1.1'
option netmask '255.255.255.0'
option ip6assign '60'
root@OpenWrt:/# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether 38:94:ed:7e:19:60 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
inet6 fe80::3a94:edff:fe7e:1960/64 scope link
valid_lft forever preferred_lft forever
3: lan1@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:61 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
4: lan2@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:62 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
5: lan3@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master switch state UP group default qlen 1000
link/ether 3a:94:ed:7e:19:63 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
6: lan4@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:64 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
7: lan5@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:65 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
8: lan6@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:66 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
9: lan7@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:67 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
10: lan8@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:68 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
11: lan9@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:69 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
12: lan0@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 38:94:ed:7e:19:60 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
13: lan11@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:6a brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
14: lan12@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:6b brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
15: lan13@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:6c brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
16: lan14@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:6d brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
17: lan15@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:6e brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
18: lan16@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:6f brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
19: lan17@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:70 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
20: lan18@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:71 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
21: lan19@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:72 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
22: lan20@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:73 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
23: lan21@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:74 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
24: lan22@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:75 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
25: lan23@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:76 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
26: lan24@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:77 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
27: lan25@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:78 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
28: lan26@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:79 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
29: lan27@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:7a brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
30: lan28@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:7b brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
31: lan29@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:7c brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
32: lan30@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:7d brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
33: lan31@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:7e brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
34: lan32@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:7f brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
35: lan33@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:80 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
36: lan34@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:81 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
37: lan35@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:82 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
38: lan36@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:83 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
39: lan37@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:84 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
40: lan38@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:85 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
41: lan39@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:86 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
42: lan40@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:87 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
43: lan41@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:88 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
44: lan42@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:89 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
45: lan43@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:8a brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
46: lan44@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:8b brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
47: lan45@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:8c brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
48: lan46@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:8d brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
49: lan47@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:8e brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
50: lan48@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue master switch state LOWERLAYERDOWN group default qlen 1000
link/ether 3a:94:ed:7e:19:8f brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
51: lan49@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master switch state UP group default qlen 1000
link/ether 3a:94:ed:7e:19:90 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
52: lan50@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master switch state UP group default qlen 1000
link/ether 3a:94:ed:7e:19:91 brd ff:ff:ff:ff:ff:ff permaddr 80:00:4c:0c:00:00
59: switch: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 38:94:ed:7e:19:60 brd ff:ff:ff:ff:ff:ff
inet6 fe80::3a94:edff:fe7e:1960/64 scope link
valid_lft forever preferred_lft forever
60: switch.1@switch: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 38:94:ed:7e:19:60 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.1/24 brd 192.168.1.255 scope global switch.1
valid_lft forever preferred_lft forever
inet6 fdfb:c33b:902::1/60 scope global noprefixroute
valid_lft forever preferred_lft forever
inet6 fe80::3a94:edff:fe7e:1960/64 scope link
valid_lft forever preferred_lft forever
I don't know why I didn't have network connectivity. I'm connected to lan1, port is up but my laptop or my switch are unable to ping each other (directly connected with a network cable).
Also, I'm connected to lan2 only, so, I don't know why lan1 is up as nothing else is connected to the switch. 
Do you have any clue ?
Thanks again for your help.