I've found flatpak to have taken several design decisions that almost seem tailor made to make it hard to use. I use an app launcher as I use I3 to run apps, except I can't use it for flatpak because it doesn't just make stuff available on the path, I'd have to make a wrapper script or something at which point I've decided to use another app or package. It also had an issue where everytime I got a gpu driver update it updated every single flatpak fair enough but it kept all the old versions! It was using double digit percentage of my disk for no reason, and the response on issue for this on the repo was just this is intended behaviour.
If it wants to get mass adoption they need to work on letting it get out of the way of people trying to use it.
I'd love to be able to do this as well. Or subscribe to a whole instance somehow.