It sounds unfair that in juried competitons an artist who can pay plenty of money for fancy slides has a leg up. A strong visual reproduced in a mediocre slide is, all of a sudden, a mediocre visual. Put yourself in the shoes of an art judge. Hundreds of them. Through experience and sheer brain power, you can eliminate most of the 258 classic mistakes that makes an art repro slide less than perfect.
Glare is a problem under sunlight, yes.
A keen eye will find a sweet spot where there's minimal glare but the shadows are not too angular and pronounced. Combatting glare will be easier for watercolorists and the like, toughest for acrylicists. For cutting glare, polarizing filters are not the silver bullet some shutterbugs claim. To find that sweet spot, stand at your camera position while a helper waggles the painting. Direct your helper to stop moving when the hot spot appears at the edge nearer the sun.