I would recommend Linux Mint. Yes it’s faster to update than Debian, but it doesn’t push the envelope nearly as fast as Fedora or Arch based distros.
Linux mint is just super easy, user friendly, you could use Mint without ever touching a terminal if you wanted. BSD would be a great pet project to fiddle with, but if you’re looking for a rock solid backup machine with zero fuss, Mint is perfect for that.
Love it when people speak with authority and are confidently incorrect. Eugenia is right.
You could potentially use flatseal to grant the flatpak the necessary permissions, and you might find out what those permissions are by looking for other users experiences with the flatpak version.
Or, you find the .deb file and it installs natively without being sandboxed. OR, you can find a PPA repository for it, load said repository and install your software.
But those things require learning a little. Linux rewards self starters who can use a search engine and forums. Hope this maybe points you in the right direction.