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
A veteran there, so it’s not much of a replacement for me, but many Redditors seem to have migrated to Tumblr.
Most of the subreddits I was subscribed to were about Classical music, there’s no much on that topic on the fediverse, but I’ve found a forum and I’m planning to join in.
Yeah, I’ve been thinking of moving my blog content to Tumblr. Not sure how much I’ll actually use the platform because I’m not a huge fan of the conversation formatting but it seems like a good option for dumping long(er) form blog writing.