So Steam didn't work. But I found a different solution: Lutris
I added PlayGTAV.exe to Lutris and manually inserted the auth parameters. I specified /usr/share/steam/compatibilitytools.d/proton-ge-custom/files/bin/wine64 as wine executable. Works fine! GTA V online session works now and I'm able to play it. Weird... Anyway, I'll have to find a way to edit the Lutris profiles on-the-fly and insert the parameters there.
Manually invoking the command does not work, so Lutris seems the only option right now (for me). Yes, I set up a fresh prefix for this. Rockstar Games Launcher also had to be installed using Lutris. Doesn't work otherwise. But that's something that I've always had to do since recent versions.
EDIT: I got it all working in a way where legendary calls my wrapper script, my wrapper script then updates the lutris game database with the new authentication parameters and then launches the game directly using lutris, effectively only requiring one command to launch GTA V.
Pinned Comments
PedroHLC commented on 2021-07-27 19:39 (UTC)
The "chaotic-aur" co-maintainer is a bot that pulls any merged PR in https://github.com/chaotic-aur/pkgbuild-proton-ge-custom-bin back to this package. So if you guys have contributions or bump versions earlier than the maintainer, please share them as PRs. The commits will keep your authorship.
Happy gaming!