Gurm My old iPhone 7 with iOS 15.7 is really comfortable. (specifically 15.7, NOT 15.8 as I've heard bad things about people who updated to 15.8).
On the other hand, my iPhone SE 2 (2020) which I bought with iOS 17.1.2 already on it is really bad for me. Immediate strain and I can totally see constantly moving temporal dithering-like artifacts on the screen through my own eyes… It's a shame because they're available for extremely cheap now and still have decently modern hardware, but the screen despite "having the same specs" is SO much worse than my old iPhone 7. Everything always looks oversaturated on my SE 2 no matter what settings I try to change. The screen also has an extremely strong "yellow tint" on all white backgrounds even when Night Shift and True Tone are turned off. Some of this may be related to "panel lottery".
My SE 2 is bad to the point where in many conditions I'd literally rather look at my OLED iPhone 14 Pro with 480Hz PWM instead (that is also still on iOS 16.4.1, importantly).
Even though the iPhone 7 also uses P3 Wide Color, I've never had any significant issue with that device at all. The iPhone 7 screen looks very stable and I can focus on it very quickly.
And of course, my "actually good LCD devices" like my old iPhone 7 are leagues and leagues ahead of my OLED iPhone in comfort, as they should be.
Because my SE 2 on iOS 17 is uncomfortable, SE 3 would probably not be better.
However, I've heard that some have had success with SE 2 and SE 3 on older iOS versions (such as 14.x or 15.x). Many people believe that something in either iOS 16.5 or 17.0 and later has significantly worsened screen quality especially on LCD devices. Unfortunately iOS devices generally are impossible to downgrade.
To the above, I have some evidence that likely proves this is actually true — on my iPad 6 that still has iOS 15, I can actually see VERY visible banding in many areas of the UI, especially on drop shadows and transparency effects in Dark Mode.
However, on my SE 2 with iOS 17, I cannot see banding anywhere at all.
This is pretty solid evidence that either my SE 2, or iOS 17 itself, is using much more temporal dithering than my old iPad that is still on iOS 15.
The iPhone 11 is also not entirely safe either, as it has the same specs as the SE 2. I remember reading that iOS 17 worsened screen quality even on the iPhone 11, so if you want to try an 11 it would be ideal to find one on 16.4 or earlier.
For what it's worth, I know someone with an iPhone 11 and it definitely looks much more comfortable and relaxing than my SE 2. However, I'm pretty sure theirs is likely still on iOS 16 and hasn't been updated to 17.
I've never used an iPhone 8 so I can't speak on that. Based on what a lot of other people have said here though over the years, it seems like the iPhone 8 is most likely the last "truly safe" iPhone.