A bunch of friends and I once toyed around with the idea of replacing microblogging through ActivityPub with one RSS feed per user. To then display them in one combined feed, I just added all of them to a category in my RSS reader, the way I already did with topics before. One of my friends' RSS reader didn’t have that option (grouping by category yes but no combined feed) so they instead combined the feeds into a single RSS feed and added that to their RSS reader. In the end it was kinda finicky so we went back to ActivityPub.