That’s interesting about the ipad, i haven’t personally had the issue myself but a few things to consider would be brightness, taking short breaks where you look towards infinity and close your eyes hard then again look far away and repeat a few times (i learned to do that in lab when i would be on the microscope for long periods) also consider getting a screen cover like paperlike - they matte out the light a bit could help. Final consideration, might be worth an eye checkup - sometimes it can be a sign of something
As for line/circle exercises definitely keep at em/ and doodling a lot. Experiment with grips, hand position, ghosting, propping up the tablet (or increasing its angle) and using your shoulders
Cheers!