WELCOME to the Forum...
I think you are going to have issue with the saddle fit same as you probably did with your eventing saddle.
Your horse is not built with symmetry. She is noticeably larger in the right side shoulder/wither area. With a deficit in her wither pad area too right side.
A saddle pad with shimming think is what you will need to search for and recognize that as you ride and build her muscling she will need to be re-fit periodically.
As for a high-wither.... a cut-back pad might give you wither clearance if it is a built up front but that is not going to address the lop-sided...and that to me is a problem.
I'm not positive but think a hoop tree is what those with horses truly razor sharp and high withered find decent fit with.
And not sure but the horse may need a evaluation and possible alignment being when I look forward from the tail at the wither mark you have a appearance of bent/twist...is that the mane though IDK...
But my gut is heading toward something may be out of whack and if corrected much of your problem may disappear.
I look forward to reading others opinions on options special for the western saddle.
🐴...
