For anime there’s not really activity for episode or Manga chapter discussions, and found that the My Anime List forums ended up filling that void. https://myanimelist.net/forum/
For buildapc checked out pcpartpicker and found they had a forum with people to help and being pcpartpicker the layout of parts is really nice when people suggest them to you https://pcpartpicker.com/forums/forum/27-create-a-part-list-for-me/
For more casual stuff like pics and memes found squabbles has pretty good activity https://squabbles.io/
For NBA stuff found realgm forums with lot more activity for individual team boards https://forums.realgm.com/boards/
For video game deals found this forum https://www.cheapassgamer.com/forum/10-video-game-deals/
It was nice that forums are still alive once I finally tried to seek out communities outside of reddit after all these years
Always been a fan of Hacker News at Y Combinator: https://news.ycombinator.com/
And we built and maintain a Reddit / Hacker News alternative at Cyber Report focused on cybersecurity: https://cyber.report/
Edit: in my own personal interest niche category of Archery, and this is interesting: https://www.archerytalk.com/ Archery Talk was always much bigger and popular than the subreddit for archery. I wonder how many niche topics always had a non-reddit, category dominant forum. A lot probably.
I’ve been seeing that site come up a lot in my searches lately.