There's a need for in-sub filtering too for non too specific subs. It's said to be possible with the Reddit Enhanced Suite thing ? but I never used it. I just echo what people say.

Sure for very narrow subs, with adequate moderation manpower, the filtering can be done this way.

But you can accomplish the same thing by creating new more moderated subs. That's why we have /r/AskHistorians in addition to /r/history. If the sub isn't specific enough the solution shouldn't be to falsly and silently fragment the sub's users via user-side filtering, it's to explicitly split off a different community that behaves the way you want. In that way there is no "non too specific sub".

If you participate in a subreddit, but filter out 70% of the content, in what way are you participating?

Sure, but things are wavey and blurry, you can't always foresee if a dedicated sub should be created, sometimes it's in waves.

True, but if your users are filtering out parts of the conversation you'll never reach a point where it's obvious there should be another community created, you'll just have multiple communities talking past each other.

Sometimes there is a mix of fluff and insightful comments in the same subreddit, or even in the same comments thread.

Which is generally what most people like. But if you only want the insightful, in-depth comments you're more likely to get them in a subreddit dedicated to those types of comments then randomly in a more general subreddit. There are even subreddits dedicated to finding those few random insightful comments out in other more general subreddits, if that's really what you're looking for.