I have a similar story and one thing that helped a lot was simply switching from dark color scheme to a light one.

I don’t know the actual science behind this, there are controversial opinions on this topic, but I definitely sensed that dark color scheme was making my astigmatism much worse. It’s almost completely gone after I switched (and that’s the only thing I did) and I can work for much longer periods than before. I eventually switched to light mode on my IPhone as well.

I suggest you to give it a try. Btw, I use papercolor scheme, which is great