Thanks, awesome answers all around!
Regarding another distro, I do use CachyOS on my desktop so that's definitely an option, but for the time being I want to give SteamOS a fair shot before I go and install a different OS on it. There's every chance I'll cave and install CachyOS within a month or two.
Don’t bother messing with the system files if possible. Try to use an AppImage or Flatpak instead, [..]
Yes, definitely; I was going to do that anyway. There are only a few corner-cases, like the PWAs For Firefox that I linked in the OP, where this isn't possible or might require a lot more fiddling than I prefer. But in almost all cases, I can work with Flatpaks, AppImages, or through a web browser.
Thanks for answering, I was honestly not sure I'd get much good info here but you proved me dead wrong :)