Edit: Pipewire is a new system AND a "drop-in" replacement for any of PulseAudio/Alsa/JACK (just add whichever package you want and it will replace that particular audio stack with Pipewire's replacement)
For the second -- FFXIV works flawlessly _once you get it set up_, but there's a couple tweaks to deal with to get there -- the launcher in particular (the default/"modern" launcher doesn't play nice). Games with separate launchers in general seem to have more issues/be more finicky in Proton. I don't personally use ACT, so I don't know if there's any real issues with setting it up, but I know others on Linux who do so it's at least possible. For your reshade concerns, GShade[1] comes with a linux install script that I've had no issues with.
[1]: https://gposers.com/
Also, do things that inject overlays work under Proton? I occasionally use RTSS to monitor frame times and whatnot, and in FFXIV I use a combat log parser that adds an overlay for DPS meters. And while not critical, I like having reshade available (though that's more than just an overlay).