I feel like it shouldn’t be very difficult to get along on Lemmy - assuming you’re not simping for fascism, for example.
Some very active niche communities never really took off on Reddit though. For example, the DECA game “Realm of the Mad God” - the subreddit didn’t particularly add to the community, you’d find more engagement on the game’s Discord. So maybe there’s a different platform more tuned to your niche interest.
The opposite of authoritarian extremism is probably libertarian extremism