> It may be helpful if community apps include various metadata in the feeds they provide, so reader apps can distinguish different types of posts (e.g. perhaps include a "in reply to" field for replies).
Fun Fact: The Activity Streams JSON-LD Vocabulary, which is used in ActivityPub, started, a long time ago, as a metadata extension for Atom (and RSS) feeds.
https://activitystrea.ms/specs/atom/1.0/
Additional Atom back in 2005 provided a minimal mechanism for copying feed items from one feed into a new feed – the "social feed" in your proposal – by specifying an element which references the original source feed.
https://www.rfc-editor.org/rfc/rfc4287#section-4.2.11
Over the years people have put a lot of time and thought into distributed social media APIs and related specs and are still doing it. Current efforts like ActivityPub and the Indiewebcamp stuff are a direct descendant of some of these specs.
(If I were you, my first thought would be to simply read, what has happened in the last 20 years. At the very least it can never hurt to know, what other people found worth doing.)