I’d try a Duett Fidelio if you can find one, I used one on a similar mare for years with one of those shaped girths. The saddle I bought was a tad wide for her so I got one of those tapered white foam pads to fill out the front and rode in that for the first year until her topline filled out. You can get an upswept panel for a short backed horse if needed too if you order a new one.
The Eq saddle science is another great suggestion for hard to fit horses.