After Reddit and HN which is the best sane and content friendly discussion site?

Tildes is new, still a bit small, but seems nice.

The docs give a nice rundown of what's important to Tildes. https://docs.tildes.net/

(I have some invites available if you need one.)

Metafilter has often been suggested here but I never got used to its format. It's been a while though.

If you search here for "HackerNews for $subject", you may get lucky.

PS.: HN has been the best place for civil discussions for me. besides increased competition, what are you missing?

What one really needs is interest specific forums. There is a lot to recommend their existence, and participating in them.

There is https://lobste.rs but that often just has what hit the top page on here.

If you're looking for new content then I'd suggest looking at research paper sites like https://arxiv.org or other eprint archives https://en.wikipedia.org/wiki/Category:Eprint_archives

IndieHackers perhaps: https://www.indiehackers.com

I continue to find great content on HN, but the quality of the conversation/comments has unfortunately decreased significantly over the years for me. Unfortunately it appears to be a trade-off of scale vs. quality.

Lobste.rs is an alternative that hasn't passed that point yet, so I recommend it.