nostr:npub1sdkl2zyvnuc045tn453k2eyrg2ezkxu3qqnkgh3uejr8y8pd7duqpr0kgj 

I hear you. I feel you.
And this is what I have done the whole day.

You write 
And the one feature I truly miss are the "Collections".
“I am not sure if #ActivityPub would even support such a subdivision.”

"Collections" are the heart of ActivityPub, see "Core Types":
https://www.w3.org/TR/activitystreams-vocabulary/#types
It is just that “some software” is not conformant.

You define your RPG/Politics/Science Collections as 'streams' in your Actor Profile https://www.w3.org/TR/activitypub/#actor-objects 
It is just that “some software” ignores this.

When our thing is ready, I'll announce it …
¯\_(ツ)_/¯