@PlutoniumAcid first of all, you would go by most active not biggest subscriber numbers (which you can't actually see accurately from within your own instance), and not everyone even does this, let alone joining everything.

Recently I made a multi for news communities so I could get an overview, and it's definitely not all the same people in all the big ones (my instance lets me see the names of upvoters). People join what feels right.

I don’t want to bother posting on all the damn sites.

I don't get what you mean by this. I post all over the fediverse but I do it all from right here on kbin! That said, there's no need. Why not just post in the communities you like most? It's no different to reddit in that regard - you don't feel pressure to post in all the subreddits, do you?

Eg outside my own instance I like the big movies over at .world but when it comes to global news I prefer .ml, but for sciencey stuff I tend to sub on mander.xyz.

but it’s juuust shy of being mainstream enough for the average Joe

True, for now. Full disclosure, I don't particularly want reddit to move here just yet. It was getting too full of people who sound like my racist aunt. The fediverse has a feeling of chill still, people contribute because we're having fun building something new, there's no algorithms, shill armies, or enshittification.

I've been on the internet since the 1990s, so I've seen things rise and fall, and I think the future of social media is federated. I get enthusiastic about it, but of course it's not ready for everyone yet, and realistically most older people may never even get here. But now I've discovered it, I'm never going back. :)

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.
If no one knows, it probably doesn't exist and you could make it yourself if you were so inclined.

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.

At least on kbin, you can search for magazines(subreddits) either by names, or by names and descriptions, and then search only local(to kbin.social) or across the federated servers as well. So its really quite seamless.