perhaps a stupid question, but are there bridges for XMPP ? My impression is that XMPP is as good as empty (I do use it mainly as a federated service). Is there still a lot of development on the XMPP side of things?

I do not understand your point with ‘you cannot change the URL’. If you use matrix as non-federated and just the only user, what is the problem that you need to change the URL when you need to set up a new server on a new URL? Not being able to change the server at the same URL seems like a logical concequence of authentication, be it for server-to-server communication as for client-to-server communication.