One of the following, as part of the keel command line (editable via GRUB), should work:
systemd.unit=multi-user.targetwill override the default of "graphical.target" – this, along with systemctl set-default, is the equivalent of "default runlevel";systemd.mask=lightdm.servicewill forbid a specific service from starting, until manually systemctl unmask'd later;systemd.mask=display-manager.service– same;rescueakasystemd.unit=rescue.targetis the equivalent of "single-user runlevel"; not for daily use, but useful when fixing broken GUI.