I use PureOS on the Librem 5 and only boot into postmarketOS (on SD card) from time to time - I’d say with Phosh it’s fine, but less polished here and there (e.g. a file picker that does not fit the screen - and due to running it from SD I could not test suspend (it wakes up with read-only filesystem)). Recently, I’ve put postmarketOS with Plasma Mobile on my SD - unless you are a die-hard Plasma fan I would not recommend it - although I run select apps (Kasts, Angelfish, Itinerary, KWeather) on Phosh, too.
If the Librem 5 is your daily driver, go with postmarketOS stable - it has less apps packaged (see Packaged In | LinuxPhoneApps), but is less likely to break across updates (and at least in some cases, packages from edge/testing still work on stable).
I would suggest to try both Mobian and postmarketOS from SD card before making the switch, and to make a backup of PureOS so you can easily go back without starting from zero before putting either of them in eMMC.
That said, what’s your problem with the PureOS flatpak repo?
I think it’s a valuable thing to have a curated flatpak repo by Purism that can eliminate issues like “if I am stupid and don’t read things I may install proprietary software from flathub”, while requiring way less person-power than packaging software to Debian. Packaging for Debian is more involved than checking and re-building flatpak manifests or packaging for Arch or Alpine - and for many apps it would require extensive backporting, as newer dependencies are often necessary (even Crimson/Debian 12 is by now 2 major GNOME releases behind).
[Addition:] I am also quite sure that if you don’t want to have any flatpaks, you will always be able to make sure to not install any by uninstalling the flatpak package with apt or, if that should be difficult, by simply removing every flatpak remote (which would then be like having apt without any repository and could not do anything).