Oh, and when I do subscribe to a community, I can only see new posts. It doesn’t retrieve older posts
I'm pretty sure that is working as intended.
I don't know all the ins and outs of Lemmy, but if it behaves at all like the federation of Mastodon, it's not going to start pulling in old stuff or pulling in stuff that nobody is subscribed to.
I dunno, the park is almost a deal breaker..