Pretty much exactly like that, yes. I know implementation would probably be complex with the way (de)federation works but it'd be a nice QoL feature imo.
Yea I did the same, but personally I like to view a singular topic at a time so it would be nice to have like a "multi-lemmy". Looks like it's not here yet but its been requested on the github.
Id seen sublemmy referenced a few times in other places so I figured it was the agreed upon term. Community is less cringe so good to know.