I agree with G&S. If your saddle is sliding around, either you’re putting it in the wrong spot, or the fit is wrong, or both. It’s hard to tell from a few pictures, but if it’s too wide, then a riser pad will actually make your horse less comfortable and your saddle more ill-fitting. Just because you are “used to putting saddles as far forward as possible” doesn’t mean that’s where it should be, and if you have to use a plethora of other equipment to try to “fix” it, then you need to start over and get a real saddler out there to look at your horse. I went through similar struggles with my boy. He changes shape depending on how in shape he is, which is extraordinarily obnoxious, and when I bought him, the saddle I had appeared to fit fine in the wither but bowed in the middle. After much researching, I decided to buy a Mattes half pad (the one you can put shims in) and cut up the shims in such a way to improve the fit. After he’d been in consistent work for 4 months (and monitoring saddle fit), I re-evaluated the fit again. No bowing, but now it was too narrow through the withers. I wanted to get a saddler out, but none are in my area, so I sent in photos, videos of him moving freely and with me aboard, and good old chalk-covered sheets. Long story short: I bought a new saddle, one with XCH, so that I could alter the fit for when his shape(s) were different. Best investment ever. He moves SO much better (and probably feels better!) and you can’t put a price on that. My $0.02? Find a reputable saddler, and go from there. Your current saddle may even be able to work for your horse with a little re-flocking or some other adjustment. A good saddler would know. He/she would also be able to tell you where your saddle should sit (this depends on their spine and where their shoulder starts and ribs end), where your girth should be, etc. You might be surprised.
"Gentle in what you do; firm in how you do it." -Buck Brannaman