This isn't a guide, just something i think may help. To install Steam on an Arch-based distro in most of the cases a simple sudo pacman -S steam will do just fine.
The installation will ask you to select a valid vulkan package from a list. And in most of the cases that's just fine... most of them.
Then you have your very "picky" old nvidia GPU which works only with a specific old nvidia driver and if you try to install anything else, there will be a conflict. Now you can try to remove the old (working) drivers and try your luck. But looking online i find a simple way to skip this passage and install Steam.
sudo pacman -S steam --assume-installed lib32-vulkan-driver
I haven't had one personally on two different systems with AMD cards.
Were you on a fresh install when you upgraded? I imagine there might be issues if there's nvidia stuff loading on a system that no longer needs it (pure speculation).
Edit: I see some posts about dkms issues on AMD that look related to ROCm support. You no longer need to manually install amdgpu packages for this (but you used to), so that may also be a factor if you have conflicting packages. Probably depends on distro somewhat.
Oh. I was referring to the Nvidia dkms. I had absolutely no issues with the upgrade/changeover to AMD. Just put the new card in and it booted right up; didn't even have to uninstall the Nvidia drivers.
Ah ok cool, I misunderstood