Keep in mind that just 5 years ago, you could not read any keyboard screen outdoors. In this respect, keyboards have really changed when it comes to displays. Additionally, TFT and Supertwist technology now make it so you can see the screen at just about any angle, and with greater clarity and brightness. Not something I would worry about at this point. Also, when performing outdoors, most players set up somewhere in the shade. Other than marine devices, such as GPS/Plotters, it is nearly impossible to see the display of any device in direct sunlight. Years ago, I used to carry a large, black umbrella with me that attached to my keyboard stand to provide shade for outdoor jobs when there was no shade available - it was a necessity. Also, the vast majority of players rarely perform outdoors, too hot, too buggy, too easy to get caught in a sudden downpour, and more often than not, too many drunks with drinks in their hands who want to talk with the musician or DJ.
All the best,
PSR-S950, Bose L1 Compact, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, Morgan 33 Out Island Sailboat, and lots of other silly stuff!