I think there are very few realistic war movies. There are also very few great movies about war. Those are two different things.
In war films, drama takes over. If you want to understand the battle, the film Gettysburg won’t hurt, but you won’t know the details without doing some reading. Print is more dispassionate.
Walking the ground helps, too. If you decide to do that, let me know, I live nearby. I’ll show you my great-great grandfather’s name on the Pennsylvania Memorial. He was at Fredericksburg and Chancellorsville before he fought at Gettysburg. Survived it all. I’d love to have a chat with him. :)