The whole "Debian 12 doesn't support the legacy 390.x nvidia card driver" really threw me for a loop when I upgraded to LMDE6 (which is based on it).
I occasionally play older games and Nouveau wasn't suitable at all (for me anyway). Go to find and install the legacy driver. Oh. Not in the repository. Sinking feeling.
Had lots of fun trying to get nvidia's own installer to work. Can't install it if Nouveau is active. Can't uninstall Nouveau without another driver available. Catch-22 much??
However, I am stubborn and can work without a GUI for a short while if I absolutely have to. So I did. And now the legacy driver is installed.
Of course, the legacy driver .deb is (still) available in Debian Sid (Unstable); Had I not convinced myself it wouldn't have worked (unfulfilling/unsuccessful experience trying to install other "future" software previously), I might have tried that.