DJTaurus I've had this setup on my iPhone 16 for a few months and it changed it from painful to ok for me:
Brightness is permanently set to 100%. I then use the accessibility shortcut (triple-click power button) to toggle 'Reduce White Point' at 100%, to change the effective brightness.
Greyscale colour filter set to 50% (I can toggle this using the control panel but rarely do). Full grayscale isn't as good for eyestrain for me, and makes the phone more difficult to use.