I know the issue well!
Over the years, I’ve enjoyed reading on an iPad Mini the most, because of the form factor, and because it felt most comfortable.
Taking frequent breaks, say every 10 minutes, is smart, and recommended by optometrists. Even something as simple as staring into space or at a distance for a few seconds every now and then, refocuses the eyes, and is a big help. After an hour, do something else away from the device IN THE REAL WORLD!
Your eyes are drying out, hence the tearing. (Sounds counter-intuitive, but that’s what several eye doctors have explained to me.) The eyes dry out somewhat due to a lack of motion and the fixed focus. They compensate by tearing up.
Changing the text and background colors can make a huge difference, too. Amber text (or deep blue) on a black background works well for me. (Dark mode may help on recent devices.)
I do most of my ebook reading in the middle of the night on an iPod Touch 6G. Some during the day on an iPad Air 2. But during the day, I find it much more comfortable to read “real” books (with decent print size) under natural, good light.
By the way, do you have the same problem when web browsing or, say, reading MacRumors? Scrolling text and jelly bean effects on an iPad (e.g., the latest Mini 6 in portrait mode) could be factors, too. My old 2015 Mac Book Air was horrendous when I scrolled through long vertical news stories or documents. A retina MBA in inverted screen — and jumping screens rather than scrolling — solved that problem.
I also have *automatic* page turn, with no effects, set up in my ebooks. Hence, no curl, no slide, but instant switching to the next page. The curling page effect is cool, but tires the eyes.
Hope something in all that helps!