Earlier quoted context omitted.
Do you have the brightness on your monitor set really high or something? I frequently use macOS on a projector , it doesn't quite fill my wall floor to ceiling but it comes close. I don't use full screen often, but I do it occasionally as a focusing strategy, and it's fine.
Projectors are way easier on the eyes than monitors though. You're shining a bright light on a wall, which you are looking at. With a monitor you are shining a bright light at your face, while staring directly at the lightbulb!
If you're using a monitor in the dark the way you use a projector, you should turn the backlight down. If you're using it in a well lit room, the brighter backlight should have less of an effect.