Mastodon is focused on posting on your personal account’s page and doesn’t really support groups AFAIK. Lemmy does have groups (communities) but doesn’t support personal posts. While I get that the two mimic their proprietary counterparts and this defines their functionality, as a self-hoster I’d like to host just one service that would combine the functionality of both Mastodon and Lemmy and potentially other Fediverse platforms.
So what is the platform that implements the most of ActivityPub features?
If friendica is interesting to you, you may give a try to (streams) and Forte. They’re evolutions of the friendica and hubzilla codebase, by the same authors, to turn it in a slightly different direction: namely, (streams) is a ActivityPub-centered experience and Forte is ActivityPub-only, both with honestly amazing features still not found elsewhere. They should both be looked at by more people (unfortunately their style is dated)
https://forum.statler.ws/channel/streamsgarden