From what OS, and for which activities?
Generally, I would advise vibe checking with a Ventoy USB and a live .iso. See if you find your marks, and can do basic stuff. Ventoy will allow you to try different distros in relatively quick succession
There’s a note on the Flathub page that it requires permission to your home folder, so this should be granted automatically. Maybe they made an update since you had the issue?
In most cases the sandboxing should not require user intervention. Apps can either use the native file picker (which gives them access to selected files) or list which directories they want to access in their manifest. If an app tells you to select a file by path-in-text-input or homemade file picker, but doesn’t have permission to the relevant directories, that’s a config issue on the packager’s side.
You should install Signal as Flatpak. It should be available through the app store if you are on Mint. Otherwise see https://flathub.org/en/apps/org.signal.Signal
Do not consult companies websites for how to install on Linux if you can avoid it, they will nearly always break your setup. Just head for the app store and click the big green button.
You might have issues trying to install clamav on Bazzite as it is an immutable distro
Antiviruses are rarely used, I wouldn’t install them on a newish distro for a non-tech user. It sounds like it may cause more issues than it will intercept
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