Setup of WDS - Xiaomi Router 4a Gigabit

Hi all,
I am new to OpenWRT (but have already some experiences with ddwrt) and want to setup a WDS.

My Setup
2x Xiaomi Router 4a Gigabit
running OpenWrt SNAPSHOT r15237-fca0eb2d92 + Luci instsalled.

The target should look like this:

One AP (aka Markus) is connected via WAN port to my LAN.
Second AP (aka Manuel) is on a different floor and should connect via WDS to Markus.

Clients should be able to connect to any AP via wifi or lan.
DHCP should be done via the Fritzbox (which is connected to WAN-Port of AP.Markus).

I am not so sure if my setup is correct so far.

What I did:

AP.Markus:

Interfaces LAN:
Static IP: 192.168.178.25
IPv4 mask: 255.255.255.0
Physical Settings: Enable STP
DHCP Server: Ignore interface

Wifi Setup:
Operating Frequency: Mode N, Channel 11, Width 40 MHz
General Setup Mode: Access Point (WDS)
Network: all devices/ports in lan.
Security WPA3-SAE.
Country Code: DE

DHCP and DNS:
DNS forwardings: 192.168.178.1

AP.Manuel:

Interfaces LAN:
Static IP: 192.168.178.26
IPv4 mask: 255.255.255.0
IPv4 gateway: 192.168.178.1
IPv4 broadcast: 192.168.178.255

Physical Settings: Enable STP
DHCP Server: Ignore interface

Wifi Setup:

On Wireless Overview, clicked SCAN on my radio0 (2.4GHz).
And selected "Join Network" of the AP.Markus SSID
Entered the WPA passphrase and assigned fw zone lan.

Operating Frequency: Mode N, Channel 11, Width 40 MHz
General Setup Mode: Client (WDS)
Network: all devices/ports in lan.
Security WPA3-SAE.
Country Code: DE

DHCP and DNS:
DNS forwardings: 192.168.178.1

Interfaces:
> I am not so sure, if I have to bridge the new interface (via Network->Interfaces) and I am not sure on which inteface of AP.Manuel I have to enable STP.

I get a connection but only Signal -50dBm - 6.0 Mbit/s 20MHz (RX and TX).

Questions:

  1. Is my setup correct?
  2. On the WDS Client, how do I have to bridge the interfaces and where to I have to enable STP?
  3. Why is my Tx/Rx so low, overthought the signal is sufficient for more? I have two Archer C7 right next to the Xiaomi on 2.4 Ghz (but channel 1) and they have >200Mbits. Why do I have a channel width of 20 MHz overthought I configured 40Hz on both devices?
  4. For performance, should the end user clients join the WDS Wifi or should I add a new one?
  5. Why is the wifi showing b/g only?

Thank you for your help

*I want to move from DD-Wrt to OpenWRT as the WDS stops working on a random basis and I am not able to figure out the root cause.