That's a very power-intensive way of doing it. Sunlight is about 120 watts per square meter[0]. My 14-inch laptop (a Thinkpad T430) display has an area of 0.0543 square meters, so in full sunlight it is illuminated by 6.5 watts of light.
White LEDs are currently approximately 25% efficient (and have a limit of 44%)[1], so my LCD display needs to use 26 watts of electricity to be as bright as direct sunlight landing on it.
My laptop's backlight can't actually do that, but if it could, it would exhaust 70 watt-hour battery in less than 3 hours, without accounting for any power used by the CPU, network card, RAM, or hard drive.
It's not that simple. I cannot explain the physics/physiology of it with any authority, but as someone who suffers from severe strain and headaches with LED (and some CCFL) backlighting I can you it's feeling like something to do with direct vs reflected lighting vs some brightness or contrast metric. Looking at E-Ink or print on paper in a variety if lighting is generally fine (as long as it's bright enough for a normal person to read..obv too dark and different strain occurs)...but an LED backlit LCD even on low settings (so maybe 10% of those 500 nits) results in problems. Stare at a lightbulb vs staring at a white wall that bulb is shining on for a simple illustration. I don't believe it's polarisation either since wearing polarized sunglasses doesn't result in strain and just like an LCD polarizes the light that reaches your eyes, so do the glasses.
Another option is to place the back of a plastic screen towards the sun, using it as a backlight.