Yeah, with the way the ActivityPub works right now I imagine the only working solution would be a UI Wrapper that allows you to login at all the different instances you wanna use and get a somewhat unified UI for them.
I wouldn’t be surprised if it would end up about as unified as the Microsoft 365 admin centres though. (That is, not very).

The other option I can think about is if someone designs a backend database that can be used to unify multiple services run by the same admin to allow the same login for all fediverse services hosted by that admin (f.e. mastodon.world, lemmy.world, friendica.world using the same user database and requiring only a single sign on) but that wouldn’t truly solve it either if we talk about a consistent UI.