I wish nomadic identities were a thing. It's a concept I found out about recently, where you could have one Fediverse account for all services, and just login to whatever you want with that preexisting account. Sort of what you describe as a Single Sign-On approach.

What I think is cool about this way of doing things, is that you could host an ActivityPub server, and login to any instance, even if it's not your own. For example, I could login to lemmy.world or mstdn.social with my sopuli.xyz account. Or at least that's what I understood.

It would probably get me to use the rest of the fediverse, tbh. Right now I only use Lemmy.