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 did address the point. My conclusion is it cannot be done reliably by an automated system and that we have the choice of blocking manually or defederating.
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.
Then I already mentioned why it cannot work reliably: "how would you make the difference between useful posts and spam from the same mag? You can't, it's just too much content to sort out."
So you want the insightful posts being hidden from a mag because someone else in the same mag spammed memes? Because that's what you will get. Sorry but I don't want that. I want to be out of this meme bubble. We are slowly encountering the same problem than reddit had.
I dislike the idea of "federations devoted to a topic"
Nope, I did not say "devoted to a topic". I said "In the end it will come to a federation made of instances with the same moderation policy."
I said "moderation policy", I didn't say "topic". I said that hobbyists would start the process, but they will probably federate with other hobbyists from other domain as long as they share the same moderation policy.
/all will soon become unusable anyway. It was great at the beginning, but the more people join and the more it will become unusable. You didn't really think that all instances would always be federated, right?