Good point! I was thinking more like spec authors should add a compatibility layer on top of ActivityPub (which I don't know much either) where clients can subscribe with different synchronization styles, something abstract like mastodon-like with N characters max-length, or lemmy-like, or diaspora-like, or pixelfed-like... This way lemmy developers can decide what makes sense to expose in each sync mode or even stay hidden for some types of social networks...

The idea is being efficient with memory and network resources and don't sync anything not required. I think it would be a little bit like your latter proposal but more over-engineered :D