I don’t like this kind of community/user instance because 2 instances have to deal with the same problem. E.g. a rogue user can troll on most community instances until they are banned by their user instance.
The instance fragmentatios is not as big issue as it’s quite easy to create new accounts. There was a thread about this some days ago here, I also use different accounts on different instances for different topics.
Now when a user reports a troll, the report goes to the moderators of the community. But in special cases the admins of the user instances should deal with banning. So the admins of the community instances have to deal with reports, but the solution is at the hand of the user instance admins. It’s the same as dealing with users from other instances, but an edge case.
My recommendations would be something like this: (I’m just a random user, so it’s just my point of view)
This would be useful for you and other admins, because you would have to admin much less number of instances. They would be still considered small instances, compared to big one, so you still not at the “too big to fail” level. For users it would help community discovery, there are overlap between followers of similar topics, e.g. I have friends who follow both European football and NBA at the same time, I read both selfhosting related topics and about general tech support, etc…