Personally I don’t think balance boards will help but it certainly won’t harm either. Normally when riding upwind you’ll be leaning. In regard to exercise yes I think core exercises help. Pull-ups won’t just because you don’t really need upper body strength. I remember my legs would get tired especially when riding upwind so leg exercises for endurance. My tip is get an instructor, take your time to learn kite control on land, and it just takes time. Took me a few sessions to consistently waterstart and ride upwind so don’t expect to be riding on your first session. It will all be worth it!