Until we have a cheap test method (capture card) that everyone can afford without regrets, all we can do speculate. And this may only help to find usable external devices, not displays. I fear finding a usable display is something that can't be done other than by using it and waiting for eye strain. The requirement for any further testing is already possessing a known-good display + connected known-good device. As you say there are too many variables otherwise:
- Display light spectrum (White LED vs OLED/Quantum dot with violet LED)
- Display light flicker (PWM, ripple flicker, OLED refresh flicker)
- Display pixel inversion flicker
- Display temporal dithering ("FRC")
- Driver/OS/GPU temporal dithering
Did I forget anything? It's too much already. If you buy a device with integrated display and OS (TV, tablet, smartphone), it can have any and multiple of those variables set and you'll never know which one it is that causes your eye strain.