You’ve just reminded me of something that used to happen around 20 years ago on smaller forums which is “forum affiliates”, where two or more forums with overlapping discussion interests would simply agree to link to each other to drive traffic.
I’m not sure how common that ever was or if it just happened with the types of forums I would visit, but it worked and there’s nothing really similar in the Fediverse. Normally as a rule I tend towards the “stay separate” camp for communities - but something to boost visibility of related communities might at least help with the perceived drawbacks.