First challenge is to find out, what such communities could even be named. My search-fu is weak so I might only find one such community, but there are others, bigger ones. How could I find them?

Ask.
If you found one you've found people who would know about others.
If you've found none, almost every instance now has an askkbin/asklemmy/askwhatever, and someone there will know.

Intuitively I would definitely want to join the biggest

both you and OP (and so so many others who are used to reddit, and capitalism in general, but I digress) seem fixated on constant growth, and more on the size of the community, than the quality of it. I think that's probably an issue you need to resolve with yourselves, rather than try to apply it to something like the fediverse.