Are there really any options other than minio?

I might have made some of that up but thank you for catching the intent of it.

But that's not what this conversation is about.

It would be the mod/admin who moves a community to a new instance, and the users would migrate themselves to follow the community.

Neither functionality exists but I agree that users should not be moved automatically unless it's part of an SSO scenario.

Liftoff (Android) has an option to show All from multiple signed on accounts simultaneously. That might be worth checking out.

It's under active development of course and I did find it in Google Play after searching by name ("Liftoff") and scrolling down.