Now that I've been using the OnePlus 3 for over a month since my last post here, I feel safe to recommend it. I can't promise it will help you but it helped me so I think it's worth a try. I think Samsung is the only display supplier so issues like "different displays, different eye strain" probably won't happen.
You just need to stay outside of the PWM range. The safe zone is 68-255 for the stock ROM and 65-255 for CyanogenMod.
The setup guide may use PWM until you set a higher brightness when you're finally in the OS.
The adjusted minimum brightness is not that low, but to my surprise I can use it even in totally dark rooms. All my other devices and monitors I had to set to 0 brightness to avoid headaches. But here, no headaches at all. I use the device like 7 to 8 hours a day (screen-on time).
When I'm outside and its sunny, I set the brightness to 255. I think there is a difference between stock ROM and CyanogenMod. Since I'm using CyanogenMod I can bear full brightness. Or my eyes have somehow adjusted to it - can't say for sure.
Just thought I should recommend this now for I know how hard it is to find a working device. And how much money it can require. I still have some BlackBerries and my unlucky HTC One M8 lying around which I should probably sell in time before they become worthless. I think you could send the OnePlus 3 back if you don't like it. I read something about a 15 day period.
A downside currently is customers may have to wait some weeks for delivery.