What kind of issues though with what GPU? I'm using Debian testing, it all works fine with AMD and KDE Plasma Wayland session.

With AMD, always make sure to use latest kernel, Mesa and amdgpu firmware.

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.