The Steam main window did not open, although Steam itself did load in the background. I could work around this by disabling smooth scrolling on web view and some other GPU-related option (I forgot exactly).

But then there was a strange glitch on every single game (both native and Proton-based). Periodically (e.g. every ~10 seconds on some 3D games, on every screen reload on some other) the screen turned black for about 2 seconds.

Then I remembered that I had some issue when I first installed Debian 12 two years, though I forgot which issues exactly, and that I solved them by switching from Wayland to X11.

What DE? And that's with all the latest components as above? I wouldn't use Debian stable for gaming purposes, since it falls behind very quickly. Debian testing / unstable is a better idea, and even then you'd want to install latest amdgpu firmware manually potentially.

KDE, not sure what version the kernel and the drivers are, whatever Debian stable has. I think kernel is 6.12.

I'll try updating the firmware and drivers manually if I have more issues in the future, thanks.

Yeah, for sure always have latest kernel, Mesa and firmware if you are having any gaming issues. 6.12 is already old by now.