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?
@myszka
Take a look at Friendica. It offers all the features mentioned on a single platform. Furthermore, it boasts broad connectivity to other networks and is infinitely expandable via an add-on system.