How much would the experience differ if, instead of closing my eyes, I turn off the lights or have a eyemask on? For some reason I have trouble closing my eyes. I have to really force it and it's quite uncomfortable.

Best way to find out: try it.

Kind regards.

Personally, i like it best to be in a dark room. If there's light, even if i close my eyes it still shines through the lids, and everything is tinted red

They both have their uses. Laying down in the dark can definitely help relax into it, especially if smoked on top of oral harmalas/aya or lsd or mushrooms. My usual preference is eyes open, especially in nature. But the best thing to look at on DMT is another humans eyes