BTW, I forgot to explain why Android specifically. On that OS, the apps can override the function of the volume key, and most readers - including Kindle - use it to flip pages. So you can hold the phone with your thumb on volume down, and read with minimal movement after finding a comfortable position. Ideally, this also needs a phone that is small enough to be held comfortably in one hand.
There's no similar feature on iOS, so you need to flick the screen to flip pages.
Note: Check is the OLED uses lower PWM frequency for lower brightness if you sensitive for flicker. It's significant on darker situation.
Unfortunately, some reader apps won't let you dial the brightness down enough - Kindle, most notably. "Screen filter" apps help with that.