I think the ideal UX would be to be able to see one "Gaming" community, subscribe to it, and check a box for "Include Similar Federated Communities" or something, and have posts from all of them show up in your feed, without having to individually subscribe to them.

I see an important hurdle to that: moderation rules are still specific to instances. So "multis" create some side effect UI challenges. In fact I think the current UI lacks reminders about which community you are participating in.

But I really like and support your "Include Similar Federated Communities" suggestion.