> all the #fediverse implementations in the first place should've been backends first and foremost.
Yep! It seems an obvious way to go if "ActivityPub" is truly a unifying protocol.
Generic backends that afford all sorts of options for anything that can be transported over the protocol, with separate front-ends/clients (or even multiple clients) to match whatever the user wants.
Then admins pick which backends they like and users which frontends.