Wow, cool! Did you manage to figure out how to publish the app for PureOS, as you asked previously here? I will see if I can try Muscle Buddy this weekend!
I am working on it, but I am taking the slow path. After discussing on this forum and reading everything I could find on the PureOS Store, I decided I should focus on Flathub first.
I am concerned about going through all the work building a flatpak specifically for the PureOS Store and then have it be rejected.
I decided I am going to start with a generic flatpak and submit that to Flathub. The new Pantheon installer inside the AppImage makes it easier to configure the app for desktop now so beta 3 can be more easily run by a broader audience. If I can carry that feature over to a flatpak, then Flathub acceptance should be possible.
I would create a Librem 5 specific flatpak for the PureOS store derived from the Flathub afterwards.
In a nut shell, it is going to take some more time.