Have you checked out the following few communities?
!communitypromo for new communities to promote themselves as they try to get off the ground.
!newcommunities for more or less the same, just via Lemmy World.
Also an underrated community that I like to use occasionally:
!lemmy411 to ask about whether specific communities exist/are around that you may not be having much luck finding (say, due to unique naming or nobody on your instance has joined it so it doesn’t appear in search).
Lastly there’s my own little community I threw together awhile ago that I’m not sure how to grow where you can ask around for stuff similar to other stuff you’re into: