While most Flatpaks by default aren't all tuned up, you can easily set overrides for them, see `man flatpak-override`. There also exist `Flatseal` which is a GUI for doing so.

AppImage can be used with Firejail.

Firejail use is trivial to setup, we try to keep out of box breakage minimal but still strict, just `sudo apt install firejail && sudo firecfg` + a logout and back in. Disclosure: I am one of the core Firejail contributors.