I would need better pictures. Can you take one from the side by the shoulder with the saddle on and girth. Then one from the side at a stand still with the saddle on and girth. It looks like you need to shim behind the shoulder as well. She has no muscle there and the saddle can lock the shoulders when in movement because it sit’s down in this place behind the shoulder instead of being level with muscle so the shoulder can slide. When you girth up, bring a front leg up like in extension and slide your hand with the shoulder as it goes back. Feel the pressure difference and the Block I’m trying to explain. If you start here it might make everything else very different as well. I think trying to shim the rear sounds correct if you didn’t have the issue with the shoulder lacking muscle. A picture of the whole horse would tell a lot about how this horse wishes to move in general. I have lots of saddle issue’s and it’s no fun but you learn a lot.