Unfortunately, this saddle does not fit this horse in the current configuration- and may not fit the horse at all. This applies to the other saddles that are sliding forward.
This horse has a very forward girth groove, and the geometry of your billets isn’t helping. You can see the Post Ride pics have the billets vertical relative to the ground - this is where they “should” sit when the saddle is girthed and sitting in the right spot. A point billet + anatomical girth can help - just be aware that point billet use can dig the tree into the horse pretty severely if the saddle isn’t perfectly fitted. Even more than the same tree in traditional setup.
Long anatomical girths on short billet saddles can also help with this. I’m really not sure why, but I think this setup adds stability due to surface area.
I suspect you have a functionally downhill*, low withered but not hugely wide horse that has a very forward girth groove and a well sprung rib cage. Welcome to the club - my TB is the same way! He takes a very open headed, or even borderline hoop tree - French and French style tree saddles generally don’t fit him.
Whatever you do, a crupper is the same thing as using a breastplate on a saddle that’s sliding back - it doesn’t address the fact that the saddle doesn’t, ultimately, fit.
*ETA: even if the horse isn’t truly built downhill and is closer to level, if they lack withers, big shoulders, and/or huge gaits to push a saddle back, it may slide forward instead. Hence the “functionally” above. Just wanted to clarify.