Running Steam and games securely

Is there a consensus on how to run Steam and games isolated from the main system? I've seen Flatpak mentioned in some Reddit post but I'm not sure how good the separation is. Everything about Flatpak sounds like an early work in progress, but I can be convinced otherwise.

I don't trust Steam or the closed source games at all. Currently I've got a second disk with a separate system for gaming, but I very rarely have the motivation to reboot. I want to game more (and spend less time on social media) but compromising my main OS is out of the question. Stuff in the home directory should be isolated from the games. Ideally no network access too, but Steam will not work in that case.

If someone has seen a ready made guide I'd be happy to read it. Any tips would be nice too.