Hard to meet the criteria exactly but maybe poke around some of these older threads for some ideas:

Ask HN: Sites like HN on other topics?

https://news.ycombinator.com/item?id=37611708

Ask HN: What are some communities like HN?

https://news.ycombinator.com/item?id=37616919

On a skim just now one that jumped out at me was https://tildes.net