If you have the means, try printing them little by little: chapter by chapter plus any back-of-the-book material. This way you can focus on one thing at a time, tactile paper in hand, without expending money on topics you don't yet need. You can annotate it, solve problems in the margins, file it, and reserve it for reference if/when you move on to successive chapters.

Sigh. I miss having a printer.

Try borrowing a good e-ink reader to see if it helps. If so, get a good one with larger screen if you'll be using more for textbooks.

Use an iPad, or any tablet, rather than smartphones.

I also read PDFs full-screen on my high-res, high-dpi laptop.

Try these two.

I have read tens of thousands of pages in PDF. (Yes, I checked)

Almost the same thing imo. It's a screen and I already sit so much in front of screens... all the more now that Elden Ring is out.