Each wifi radio can only be client to a single access point at the same time. You need to use two radios (2.4GHz and 5GHz, a USB wifi adapter, ...), try to trunk several networks over the same wifi, or use a wire.
Can you give me more details on this? I currently have a WWAN (for SSID My_wifi_1) and WWAN2 (for SSID My_wifi_2) interface as well as one (!) LAN interface. The issue I currently have is that I don't understand how traffic from the LAN interface flows the WWAN and therefore don't know what to do next after creating 2 VLANs (not done yet, but I know how to create them - just not how traffic from them comes to the WWAN and WWAN2)
Are your wifi supposed to be connected to internet or serve a LAN network?
Because when you name them anyting with WAN in the name they get red and are internet connections. But to have three internet connections is your first problem since OpenWRT only can have one internet connection at once.
But on the internal LAN network you can pretty freely connect many VLAN to as many SSID through interfaces per radio.
thank you all for your replies and amazing support here! I finally went with the idea of @alefello which worked out to be easy to setup and needs no different subnets for the vlan and wifi network.
here are my rough notes for one vlan/wifi, do the second one just the same way:
Wireless networks -> Scan, then join
Save, in wifi settings change from client to client (wds)
Don't assign an interface for now
In Network - Switch create the VLAN
Network - Interfaces - Devices create a bridge with just the created VLAN (due to limitations bridging VLAN directly with Wifi / required from OpenWrt 21.02 or so)
Network - Interfaces add new Interface with protocol unmanaged (DHCP in my case coming from a another device) and as device select the just created vlan bridge.
Go to network - wireless, edit the wifi client and as network choose the just created interface instead of the automatically generated wwan interface. (if you don't see your new interface in the list, don't forget to scroll up/down!)
disable the firewall as we don't need it (System - Startup)
btw I could run two WDS clients in parallel / connect to two wifis (didn't test with more than two).