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 on registering on it.
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.