Well funny thing is that l'm in the same boat as you😂
Your questions are definitely valid, but i personally think the ideal platform would be windows, but there is some platforms itch io supports, it would be a great idea if you can make your game playable on many platforms just incase
also you don't HAVE TO upload a build, i think webGL should be good enough for a simple game
also it mite decrease your chances of wining a little bit if your game only supports windows, but you should focus on making the game fun & creative with Awesome Gameplay, Design, and music that with the theme and the "Geometricness" of the jam that fits your scope.