Forcing the DPI did the trick – it was on “disable” by default, so I set it first to 96, and then to 120, and settled on the latter, as it looks better on my tv/monitor.
Turns out the issue had nothing to do with switching from gnome, either. I decided to test out KDE in a parallel installation, or else I’d never know what problems were native and what were caused by remnants from the gdm setup, and the font DPI problem presented itself right after I installed the Nvidia proprietary drivers.
So things are pretty much fixed, except the font is still magnified during the login screen, which isn’t a dealbreaker or anything, but any ideas how to force the DPI for that?