You can own certain Steam games as much as you can own them on GOG, which is to say it’s completely up to the publisher/devs. https://www.pcgamingwiki.com/wiki/The_big_list_of_DRM-free_games_on_Steam

Phew, I have a 2 word username. I’m safe!

It was for me at least, only moved to Linux after spending a long time with the steam deck

HDR definitely

I haven’t played FFXIV since switching to Linux so I haven’t tested this, but it seems there’s a Dalamud plugin to have the ACT plugin working without having to deal with ACT itself. https://github.com/marzent/IINACT