Through the years, an iPhone 11 Pro, 12 Pro, and 13 Pro caused me absolutely no eye strain; where I could use each for hours at-a-time. However updating iOS on that 13 Pro (from 15.3.1 to 15.6), caused it to become completely unusable.
I currently have a 14 Pro, running iOS 16. But it causes the same level as strain (dizziness/fatigue/brain fog) as the updated 13 Pro.
So hardware-wise, the iPhones are good (at least for me). But I firmly believe it's iOS which induces the strain... and unfortunately through more calls than I can count to Apple Support - in providing evidence that their updates to iOS are causing me severe issues - they refuse to offer any assistance in helping/allowing a downgrade of iOS (on that 13 Pro). (Otherwise I have unfortunately sold the 11 and 12 Pros)
So though most iPhone Pro and Max models typically get negative feedback from us with strain issues, if you can find a model running iOS 14 or below, you may be alright.