Debian 12 KDE Shows a Black Screen Immediately After Login

I had to reinstall Debian 12 with KDE after my system drive failed. My personal data was preserved on a separate drive, which I configured as /home. Since then, KDE sometimes gives me a black screen immediately after login. The display manager works normally, and I can enter a TTY, but after entering my password the KDE startup animation appears briefly and then the screen goes black. I am using an X11 session.

Booting with nomodeset restores the desktop, so I suspect a graphics or display configuration problem. I tried resetting KDE Plasma settings and removing old configuration files, and temporarily copying the default home directory contents made the desktop work again. However, after launching an older game through Wine, the screen went black again and I had to force a shutdown. Changing the monitor from one HDMI port to another sometimes restored the image, but now restarting SDDM has left me without a display unless I use nomodeset.

I can still access a TTY while the graphical session is black. What is the best way to diagnose this and reset the relevant KDE, display, graphics, and user-session configuration without reinstalling everything? Would creating a fresh user help determine whether the problem is limited to my existing home directory, and can that be done from a TTY?