I seldom use it, but for filtering https://hn.algolia.com does a good job.

On the other hand, I wonder if there's some AI tool for tagging posts and building custom feeds.

Well, lobste.rs has tags and then you can block whatever tags you choose to block, and for HN you'd need a reader that classifies / auto-tags posts and allows blocking out tags, which you'd start doing after verifying the auto-tagging classification works reliably.

Sounds like a bit like a tall order, not sure it's "just a weekend project", would (for my purposes) probably work best as a browser extension for just the listing of posts (auto-tagged and then optionally tag-filtered) to then open them in one's browser, allowing normal native reading+posting+voting+etc.