And I think it should be based on RSS 1.0 or the Atom Syndication spec (except dump XML and use JSON or something)
But ActivityPub is the evolution of Atom/RSS, with JSON instead of XML...
Perhaps you actually want a 'Really Simple ActivityPub'? It could be designed to be compatible with ActivityPub, so a bridge won't be necessary.
I think this is doable. I think the flavor of ActivityPub we use in Fediverse is already very close to Really Simple ActivityPub, it just doesn't have a spec (yet).