If I thought any of the existing standards were winners, I would've gone with them. If you really want to get mad, you can read why I decided not to use RDF or JSON-LD: https://unwalled.garden/docs/why-not-rdf
Activitypub implementations seem to fare mostly fine by ignoring that ActivityStreams2 technically is JSON-LD, using it as "just JSON". Curious if you have any particular points against it (I can imagine a few, but would be interested in your take on it).
Edit: also, thanks!