TLDR: The miniLED screen is better than OLED on my eyes, but still causes eye strain. I suggest trying it yourself, it's definitely a step forward IMO.
So I've been using the new iPad since Friday, here are my impressions eye strain wise:
-Slow motion video of the screen shows very little (If any) flicker, nothing like OLED flicker.
-It doesn't seem to have that "shimmer" effect that I usually get from Macs.
-The eye strain from this iPad is much less harmful than Macs/OLED iPhones, it feels like a slight prickling above my eyes after some use, but no migraines (yet)
-I don't really notice any huge flicker difference between brightness levels.
I'm very sensitive to flicker, so just because the miniLED screen bothers my eyes doesn't mean you guys shouldn't try it. It's definitely a step up from OLED and the Macs IMO, I think it's worth a shot to try.
I'm going to try messing around with brightness/white point/night shift/true tone a bit more to see if anything helps, I'll post my results in a few days.