The two other "indieweb" forums I use from time to time are
melonland and
basement community. They're both much smaller and less active than Agora though, so don't get your hopes up. Melonland enforces its "friendly atmosphere" a lot more, so if you want to act like you're on /b/, it's probably not worth visiting. Basement Community has a nice simple ui, and a few cool regular posters, but most of the threads or introduction threads by users who never go on to make a second post. I wish it was more active, but it's still worth checking out.
You probably won't find these interesting if you don't share my interests, but I figure they're worth posting anyways.
Zenius is pretty much the only community hub for rhythm games. If you like rhythm games, check it out, if you don't, there's nothing there for you.
Giant in the Playground is my favorite forum for discussing tabletop rpgs. TTRPGs are relatively mainstream now, so if you play 5e you can probably find communities on normie social media, but I personally find the discussions here to be more useful and interesting.
rpg.net is the other big ttrpg forum, but it's rules are a bit more draconian, and harshly enforced. I don't really post there anymore, but if you play more obscure rpgs, it's still worth a read.