From what I’ve read Galaxy allows for downloading specific version of the games.

Might be misremembering, so take it with a pinch of salt

Surely they’ll make a native Linux client then, right, right?!

(Meanwhile; shoutout to Heroic)

Any particular reason you’re not on/tested the v590-branch? Of course it might be borked compared to 570, but might be worth a shot?

FYI:

the default, one of AMDVLK and RADV

I think that AMDVLK is shipping RADV per default now, so you might as well use RADV (for gaming, there might be other differences in OpenCL etc.), see Github/AMDVLK for more information;

In a move to streamline development and strengthen our commitment to the open-source community, AMD is unifying its Linux Vulkan driver strategy and has decided to discontinue the AMDVLK open-source project, throwing our full support behind the RADV driver as the officially supported open-source Vulkan driver for Radeon™ graphics adapters.

Why not just grab the previous PKGBUILD version that worked?

… I can’t undervolt my card…

People usually use/recommend LACT for undervolting/overlocking on Linux