Yep, that worked:
mount /dev/sda1 /mnt
cat /mnt/rbt-with-aux-for-mtd5.bin | mtd write - RouterBOOT
echo cfg > /sys/firmware/mikrotik/soft_config/boot_device
echo 1 > /sys/firmware/mikrotik/soft_config/commit
cp /mnt/openwrt-mvebu-cortexa72-mikrotik_rb5009-squashfs-sysupgrade.bin /tmp/fw.bin
sysupgrade /tmp/fw.bin
However, still no reaction from WAN. While LAN on p2 works fine WAN on p1 (the 2.5 GbE one) seems dead:
root@OpenWrt:/# ethtool p1
Settings for p1:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
1000baseX/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
1000baseX/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Auto-negotiation: on
master-slave cfg: preferred slave
master-slave status: unknown
Port: MII
PHYAD: 0
Transceiver: external
Supports Wake-on: d
Wake-on: d
Link detected: no
root@OpenWrt:/# ethtool p2
Settings for p2:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Link partner advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Link partner advertised pause frame use: No
Link partner advertised auto-negotiation: Yes
Link partner advertised FEC modes: Not reported
Speed: 1000Mb/s
Duplex: Full
Auto-negotiation: on
Port: Twisted Pair
PHYAD: 7
Transceiver: external
MDI-X: Unknown
Supports Wake-on: d
Wake-on: d
Link detected: yes
So the 2.5 GbE port does not seem to advertise 2.5 GbE and also does not react on anything being plugged in.
Curious how the SFP+ behaves... There you go:
root@OpenWrt:/# ethtool -m sfp
Identifier : 0x03 (SFP)
Extended identifier : 0x04 (GBIC/SFP defined by 2-wire interface ID)
Connector : 0x21 (Copper pigtail)
Transceiver codes : 0x10 0x00 0x00 0x00 0x40 0x40 0x04 0x00 0x00
Transceiver type : 10G Ethernet: 10G Base-SR
Transceiver type : FC: short distance (S)
Transceiver type : FC: Shortwave laser w/o OFC (SN)
Transceiver type : FC: Multimode, 50um (M5)
Encoding : 0x06 (64B/66B)
BR, Nominal : 10300MBd
Rate identifier : 0x00 (unspecified)
Length (SMF,km) : 0km
Length (SMF) : 0m
Length (50um) : 300m
Length (62.5um) : 300m
Length (Copper) : 0m
Length (OM3) : 300m
Laser wavelength : 850nm
Vendor name : OEM
Vendor OUI : 00:1b:21
Vendor PN : SFP-10G-T
Vendor rev : A0
Option values : 0x00 0x1a
Option : RX_LOS implemented
Option : TX_FAULT implemented
Option : TX_DISABLE implemented
BR margin, max : 0%
BR margin, min : 0%
Vendor SN : 2104060324
Date code : 210331
root@OpenWrt:/# ethtool sfp
Settings for sfp:
Supported ports: [ MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
10000baseT/Full
2500baseX/Full
10000baseKR/Full
1000baseX/Full
10000baseCR/Full
10000baseSR/Full
10000baseLR/Full
10000baseLRM/Full
10000baseER/Full
2500baseT/Full
5000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
10000baseT/Full
2500baseX/Full
10000baseKR/Full
1000baseX/Full
10000baseCR/Full
10000baseSR/Full
10000baseLR/Full
10000baseLRM/Full
10000baseER/Full
2500baseT/Full
5000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Auto-negotiation: on
Port: MII
PHYAD: 0
Transceiver: internal
Supports Wake-on: d
Wake-on: d
Link detected: no
So while it recognizes my 10GBASE-T transceiver, which means I2C is working, no further reaction so far. Okay, after another re-boot with the SFP+ already plugged in it looks slightly different:
root@OpenWrt:/# ethtool sfp
Settings for sfp:
Supported ports: [ ]
Supported link modes: 10000baseSR/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10000baseSR/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Auto-negotiation: on
Port: Direct Attach Copper
PHYAD: 0
Transceiver: internal
Supports Wake-on: d
Wake-on: d
Link detected: no
However, still no link or anything. But for a first try, quite amazing! Thanks, @adron!