This is likely not what you seek in terms of simplicity, but I wanted to mention in the context of earlier discussion on #ActivityPub JSON-LD versus #RDF XML.
The #openEngiadina project for a long time pursued an ActivityPub-based implementation for its open local knowledge network, but found it too complex, esp. in the areas where the specs are immature and incomplete.
So the research project switched to #XMPP/RDF and had some good experiences.
Oh, what also just comes to mind is the #Polyproto protocol. I think it may have been created in reaction to ActivityPub's complexity and issues around identity, not sure.
And another protocol in the works, inspired by AP is #Versia.