I can't speak for the more "entreprise-y" unifi stuff but the routing on the ubiquiti gear i've used is actually just FRR and if you SSH in you can do a whole lot more than is surfaced through the UI - for example, I have VXLAN running on my UDR7 right now.

It's not something I've tried but I imagine you could actually expose FRR directly and configure through that

For a long time if you made any changes through the web UI it would overwrite the config. Then you could make changes and it would save. Then they removed it. Then they added it back but it didn't always work. Not sure the current state.

I have like a ~15y history of working with their stuff.