if you want a secure device, keep things separate. use your other fancy high-end computer for sensitive stuff. if you truly cared about security and privacy, you would make the space for it and use them separately.
by hardening a flatpak app, lets say Steam for example, you run the risks of certain games no longer running properly due to limiting permissions (ptrace, ipc, shared memory, etc).
i have been gaming on fedora close to 6 years now. the reason why i chose fedora is because it has SELinux configured out the box. i have never had an issue gaming on fedora aside from the standard kernel level anti cheat games no longer working on it. for sensitive stuff, i have a laptop running secureblue.