I don't think your answer addresses what the OP is talking about at all and it's getting kinda scary that the immediate answer to every small issue people have on the Fediverse seems to be defederation. People do love their echo chambers apparently.
I want to be federated with lemmy.ml and lemmy.world and etc and I'm fine that they're generic instances. They don't need to "specialize" and I don't even think that was the point. Maybe people assumed Kbin and Lemmy would follow the tracks of Mastodon where indeed some instances were created as specialized art or technology instances, but I don't even think that's really a great idea.
But I do agree that some communities are a lot more active than others - like meme communities - which leads their posts to drown out other smaller communities. I don't think the problem is the "quality of the content" - I don't want anyone controlling the "quality" of the content either, as long as it's within the rules, because that's just way too subjective - for example, you seem to think all meme accounts are spam, whereas I don't think they are at all - I think people are allowed to have fun.
What I want, and I think that's what the OP is talking about, is simply to have a way to slow down the posts from the more active communities - kinda like how Reddit didn't show you all of the content of a very active sub when you were on your main feed, but only the hottest threads, and you'd get a mix of threads from very big active subreddits and smaller more inactive subreddits. I love the LOTR meme communities, for example, but I don't want to see all of their posts on my main feed, since it drowns out discussions from other smaller communities. All we need for that is a limit to the amount of threads you can be shown from every community on your main feed.
I dislike the idea of "federations devoted to a topic" because they get boring really fast unless your whole personality centers around that single topic. I created an account on fosstodom once since I do contribute to FOSS and like seeing discussions about it, but eventually it got boring because, naturally, no one talked about anything other than software and people didn't engage on other topics - which is probably fine if software is the one and only thing you like having in your life, but I would dare say is not fine for most individuals with a balanced life and different hobbies.