I came across this topic while searching for the same answer.
First you want to disable the display manager service, but what's it called? Find out what services are display related:
Here's how I found out which service to disable:
chris@trippy:~$ systemctl list-units | grep -i display
nodm.service loaded active running LSB: No Display Manager
Now, I want to disable the service called "nodm" (which is a cute but very confusing name):
sudo systemctl disable nodm
sudo reboot
