Update 02/05/2021: Somehow the ethernet backhaul is not being used. Initially it worked for a fairly remote router (different building), and after reshuffling things it keeps connecting via WiFi instead of Ethernet. Since the connection is “good” I didn’t bother to try and fix it, however my dad now fot a similar setup and also the Ethernet backhaul seemed to work initially and after repowering the router it switched to wifi as well. Is this a bug? On purpose? It doesn’t seem expected behavior.
After a lot of research, I decided to go for Google Nest Wifi (3 routers, 1 extension). Works really great. I added a (USB powered) switch to get more ethernet ports. The router does have better coverage than the extensions, so I prefer more routers with Nest mini over the build-in speaker that comes with the extensions.
I received a Genexis Titanium router from Caiway to allow for a bridged glass fiber connection.