Congrats on finding something that works, that's great news!
Out of curiosity, have you tried any of the following?
-Using the 13 without the protector to see if the symptoms come back
-Using the screen protector on a different problematic device to see if it prevents symptoms
-Trying different types of games to see if it affects the onset of symptoms (For example, a more graphics intensive game may cause worse symptoms)
-Messing with the brightness levels to see if it affects symptoms
You mentioned that you also tried a 13 Pro a while ago & had symptoms, I'd be curious if you'd now be able to use one if you had this screen protector on as well. The strange thing is that I don't really see anything special about this particular protector (Seems like a standard matte tempered glass protector from what I can see), I'm definitely interested as to what makes this one different.