If I understand you correctly you fully neutered the discrete AMD GPU, so it’s no different that a 13” MacBook Pro at this point.
If that’s the case are you able to see a bit of glow from the screen in a darkened room? If not are you able to press a flashlight against the lids Apple logo (a poor man’s backlight) and see your faint desktop and its icons?
You might want to see if an external keyboard, keyboard and mouse works when the display lid is closed. If it does they points to a display issue.