Basically, I suspected that disabling the desktop environment on a full Debian install would involve some combination of editing an array of config files, disabling systemd services, and possibly uninstalling certain packages…
I was happily mistaken!
To disable the desktop environment autoload on boot, run the following:
systemctl set-default multi-user.target
Simply run startx after login to start the GUI, if desired.
To re-enable (but why would you?):
systemctl set-default graphical.target
EOF