How do I get my iPad to read without eye strain?

I just went from iPad Pro first generation to iPad M4. I don't know why but I get eye strain on my new iPad M4. I'm currently having to go back to my iPad first generation. my eyes are painful only after 5 minutes but my iPhone, iMac and old iPad is no problem. Does anyone know what could be the difference? I know the screen refresh rate is higher. But I can't see anywhere I can reduce that. I've already tried turning on night mode all day and turning down the brightness and turning on the Eyestrain distance tester. It's currently not helping.

[Re-Titled by Moderator]