DepthHub & RBI were good at times.
Although the obvious answer for me is the already niche subs on Reddit for software/TV shows which have just a few hundred people there. There’d be 10 people at absolute max on here and the community would not last.
You shouldn’t have to for most instances but it can depends on each instance.
The admins of an instance can choose to defederate and that blocks communication between themselves and all/some other instances (see beehaw). In that case you’d need two accounts but its safe to say if that were to happen between most instances this place would fall apart as nobody is going to juggle between different accounts.