Development notes (not for users!):
sudo apt purge gdm3 fixes the issue of broken tty1 but that’s not something we can recommend to users due to issues, not easy.
File /usr/lib/systemd/system/gdm.service contains:
# replaces the getty
Conflicts=getty@tty1.service
After=getty@tty1.service
Problem seems to be that gdm takes tty1 by default instead of tty7 (lightdm default). That’s why gdm is getting in the way.
Somehow above issue is even triggered although rads already created the required status files for systemd to recognize that “gdm condition for startup failed”. Seems like the systemd unit keyword Conflicts= is also active even if gdm won’t start due to “condition for startup failed” (status file created by rads because “not enough RAM”).
Why gdm? → display manager - lightdm, gdm3, sddm or no display manager (startx)