AGI I bow to your wisdom and will take your suggestion. I will order a OnePlus3 (it is around $190 USD brand new) and put PA on it. If this gives me a comfortable experience, I will then order a OnePlus3T (around $240 USD brand new) and do likewise. Then we will know. Either phone can be resold easily since I won't put any mileage on them. If they differ, I will sell the 3T and get a second 3 to keep as a spare. If not, I will use the 3T and keep the 3 as a spare.
As for the iPhone XS, I don't know. My only suspicion is maybe drivers? Apple is where I first saw this problem - they changed the way they were driving the screens of Macbooks in 2012 and that was it for me. It was a weird combination of software and hardware - I had the screen on a Macbook replaced with a theoretically identical screen and it hurt to look at.
In the case of the iPhone, we've seen many times that it varies from iOS update to iOS update. This forum is full of people who said "My iPhone 5 was fine under iOS 7 but broke with 7.1 and then was ok again with 8.0 and then..."
I really have no other ideas, since the XS is not especially high-res (it's 2436x1125, essentially just wide-format 1080p) but it IS using Apple's own iteration of OLED so maybe that's the ticket? I know people think they have problems with OLED devices, but I've long thought that OLED (like Plasma) was the future. And frankly I never had problems with early iterations of OLED screens. Samsung OLEDs always kind of tweaked me out but I assume that's the way Samsung was driving them from software/drivers/OS.