To be honest, you will never find a generalized site/service that has many types of niche content like Reddit unless people coalesce to a single platform and the scale allows it.

I think it’s fine if you end up finding your niche community on some esoteric website/forum, more power to you. But for a single one-stop collection of niche communities, I think the best way to get there is just to sit tight with Lemmy/Piefed/Mastodon, honestly, because at some point those communities will reach critical mass and be self-sustaining (but not if people don’t stick around long enough for it to happen).

Of course also starting/contributing to your niche community speeds things along.