These don't offer the core UX of Discord, which is being able to jump between many communities from a single app/login.

No open thing can ever do that. By virtue of openness, there are many copies of open things whereas your requirement requires there to be just one.

I think you could get pretty close with OAuth2. You could also have the frontend be a centralized app, but allow people to host their own servers. If the entity controlling the frontend goes off the rails you still have a pretty simple exit strategy.