Other than here I'm on the aforementioned Horror Board site and also iCM. The ICM forum seems huge and unwieldy to me for some reason, but lots of knowledgeable people are there.
This place is the best so far imho.

https://www.icmforum.com/index/

It's similar to Film general on IMDB but without all the trolls of course. Lots of film nerds and not as much horror discussion, but the challenges look the bomb.