Any reason why I shouldn’t just go with Debian + KDE and install Steam?
No reason to avoid Debian unless you have hardware so very new that it requires the very latest kernel to operate.
If you go with Debian Stable, you can enable Backports for a fairly recent kernel, currently 6.5.10. You could go with Testing or even Unstable if you’re addicted to upgrading as often as possible, but chances are you won’t need to.
I’m gaming on Debian Stable with Steam in a flatpak. It works great, and is blissfully low maintenance.
At some point, you’ll probably run into people claiming that Debian is bad for gaming performance because of “outdated” packages. In most cases, those people don’t know what they’re talking about. I suggest ignoring them unless they identify a specific performance issue that actually affects you.