Bridge multiple Lede Routers using VPN and one DHCP

sorry for the late response. Thank you so much for your detailed answer.

By now have the following setup:
2 Wifis per Router: one with a unique SSID for Debugging or stationary items (e.g my Echo always connected to a router more far away....) and the other one with the same SSID for all of them.

The Routers are bridged using openVPN (din't saw your post at that time). At first I tried with disabled DHCP on three of them. Which worked quite well, but unfortunately the DHCP pushes the standard gateway to the clients. This resulted in the problem that all clients (regardless of the ap they are connected) used my internet/WAN interface. As this reduced speed and my WAN Traffic is limited, I made 4 individual DHCP server running on the same subnet with a dedicated range .4-.20, .21-.40. This works quite well. The biggest Problem was that the devices weren't reneawing their lease when roaming from one ap to another, therefore I set the leastime to 10m. So when I use my smartphone around the house, it uses my WAN connection most of the time which is okay. But Laptops which make the most traffic just need 10 min after going to another room. I can live with that. So all in all it is not a very beautiful solution, but one that works (more or less).

Things are getting more complicated as I have another VPN Client running on those Routers to avoid traffic Limitation on the WAN Interface.

Maybe I will try your solution but I will get the same problem, right?