There are many instances of Discourse. I don’t know exactly but I have accounts on a few dozens. It’s impossible to keep track of everything and many times I come across topics in distinct but no so different communities discussing similar issues, that could well be shared across instances since some of the participants are repeating themselves across discussions. It would really help to be able to share such topics across instances without having to login several times, cross-reference topics and not have a fluid discussion with interested parties.
Having ActivityPub users treated as staged users the same way emails users foreign to a Discourse instance are treated seems to be a good compromise to start with.
An RSS feed would certainly help you track ongoing topics at a single place but would not bring anything different from the Discourse Hub app, nor would it allow you to participate.