Great news, I managed to wrap the official AppImage into a Flatpak.
I made some quick testing, so far the application seems to be working as expected.
This is my first flatpak, I still have to figure out how to publish it to the official flathub. In the meantime, you can find the source code here: https://github.com/flavio/plexamp-flatpak
@ plexamp developers: I would glad to help you if you want to take it over
