Moe3224 You won't believe what's happening with me, I use windows laptop for work that is connected to two external monitors and I use an iPad Pro 11" all day too with no issues. When I connect my MBA m1 to my monitors after 30mins my eyes get really red and itchy, I can't focus on the screen when looking at text.
I wouldn't discount font rendering by macOS, which is different than Windows and Linux. This was my issue and had the same symptoms you're having. Even though most may not notice it, the fonts are really blurry, and rendered in a way that makes the eyes struggle to focus on text.
The anti-glare coating of the screen blurs the fonts even further, but the font rendering could explain why people are still having problems when using external monitors as well.
MAS-76 For now I've had to give up and revert back to my 2015 MBP which of course isn't a great solution.
Along with the font rendering, the difference in your 2015 MBP and the newer models is the resolution and amount of nits (brightness) of the screen.
I too had issues with the 2019 MBP and M1 MBP, then I switched to a Thinkpad that has around the same resolution and nits as your 2015 MBP, (as well as running operating systems that do not share Apple's techniques for font rendering) and I'm ok now.
I believe the higher the resolutions, the worse the problem becomes with Apple's font rendering, as if you leave it at native, the text is hard to read, but if you scale it, then you increase the blur. The font rendering also got worse with newer macOS updates.
As for the difference in nits, I notice the problem for me gets worse in light mode, hence I had to keep it at dark mode at all times. Keeping the brightness to full on a 300-nit screen is much better for my eyes than any level of brightness set on a 500+ nit screen.
Of course, I'm just speculating with all the above based on my experience, so don't take my word for it.