You can get a decent refurbished S8+ for around $250. I just did.
The screen brightness is even better than my old Dell Streak and the screen is slightly bigger even though the overall size of the phone is significantly smaller than the Dell Streak.
I can’t imagine a need for any brighter screen, nor a justification to compromise the additional functionality which color provides vs the Kobo. I understand that an option to overlay with skysight is coming, so one more reason to have a color screen. Not to mention the advantage of modern touch screens and gestures vs the Kobo which I believe is pressure sensitive touch screen.
I also ran it at home for 8 hours and it only used half the battery capacity. It was in flight mode and the screen was on all the time. I am not sure if it takes more battery when in flight. Hope to try it in the air soon.

Ramy