I think Mastodon is very far from standard

I think it’s much closer to standard than Lemmy and I’ve looked into it quite a bit recently. ActivityPub is unfortunately quite focused on microblogging. Honestly lemmys way of doing it is a little hacky.

As for the posts outside communities? That makes sense lemmy-wise I think. Where would those posts be?

I actually think it’s quite straightforward, they’d just be on a users page. This is actually how Reddit has also done it ever since they introduced the feature (much before they enshittified everything else).

You can think of it like every users profile being a community of its own but only the user itself can post to it. Just conceptually speaking.

That would also let you follow users just as you can follow communities.