I've not participated much with communities, but, a few that started out as S-E tribes I would find to be pretty great ones to post in with relevant content; InnerBlocks, HomeSteaders-COOP, and Natural Medicine, one that has a S-E token backing it that just recently started is the gardening community Hemp.

I feel that the barrier to entry should relate to the topic the community is about, some topics naturally have a higher barrier than others.