I think multi-communities will solve a lot of this - you can group, for example, all the movie communities together or the meme ones and get a coherent feed, so it wouldn’t really matter which you posted to.
The UI could detect sibling cross-posts and suppress multiple mentions of the same post if you’re subscribed to multiple sibling communities, maybe with a “cross-sibling post” designation. That way it only shows up once in your feed.
Doesn’t this already happen? At least within an instance, my experience is that, if you cross-post, the second post doesn’t appear in your all feeds.