The niche communities on Reddit built slowly too. Just keep posting content to these communities. Potential new users are MUCH more likely to stick around and participate if it seems somewhat active.
This whole thing relies on a tiny percentage of users that actively participate and try to enrich their communities be part of that percentage.
You can use kbin “magazines” (what they call communities) from Lemmy! They’re federated too!
Click this, for example: !AskKbin@kbin.social