After a billion saddles and dirty saddle pads, I discovered he liked a saddle that was too wide in the gullet but not excessively wide... doing the too wide and padding it to fit thing did not work for him. He needed a gap over his shoulders, not filled with pads.
Said saddle also had to be narrow in the channel and slightly more banana shaped that his back. Panels had to be extra wide and flat in profile. Its amazing I found a saddle to fit this description, but I finally did. Only took 35 saddles to find it.
Point billets were uncomfortable despite him having the conformation that called for it.
He disliked bearing any weight on the muscles around the shoulder area and slightly further back, right where tree points would normally be.
A custom fit saddle would be excruciating for this horse. I am SO grateful I didn't go that route and tried figuring things out for myself, as otherwise I would have had it in my mind that "the saddle is custom made and professionally fit and therefore isn't the problem".
He was intensely sensitive to a rider slopping around or rocking at all too... even saddles rocking or popping up in back (ie longing no rider) would get him tail tucking or cross firing. He taught me to sit *extremely still* lest I be launched into the stratosphere. And if I even thought about jumping ahead I was going over his shoulder instead.
I did discover that he preferred I mount from the off side, and that a mounting block was mandatory. Fortunately he is short and I have long legs for my height, he did best if I could just swing a leg over rather than using the stirrups at all to mount.
Girth selection was critical for him, I had to have super wide and super stable so the saddle wouldn't budge. I ended up with mohair in a wide roper-style diamond shape. Even with his driving harness, he prefers being girthed very firmly, not quite cut in half, but close. He would rather be girthed hard than have that saddle move a millimeter.
When we finally decided to start driving, I had to have his harness saddles custom made to be extra wide and have an extra large footprint, and he wears a dense wool felt pad under for extra weight displacement. We drive in a 4 wheeler so there is no weight from the shafts on his back.
I only have the one experience with my horse, so sorry if it isn't much to share, but hopefully it might be helpful in some way. Good luck.
eta: thinking back, our most successful string of riding was with a western saddle actually. A wade. Distribution of weight and keeping away from his shoulder area was just critical.
But as I said, in the end, even with all my efforts, he was never happy being ridden so I honored that.