Tucker equitation endurance is very short, and the wide tree is quite ample. Treeless saddles do not stay on top of horses with no withers.
You might have good luck with Bighorn’s Flat Topline saddle. It fits my wide mare, and my round mule. Very comfortable too.
Also look at Cactus saddles, their barrel saddles fit wide straight backed horses. I have a mare that I have purchased an used Charmagne James saddle for my very wide mare with a short straight back. I also saw an ad for an almost new Cactus roper or all around 15 in seat with breast collar and headstall included. It is a dealer, but if you want his link, private message me. I think he wanted $1500 for everything, and it is gorgeous. My Cactus saddle should be here tomorrow and I would have seriously considered this one if I hadn’t already committed to the barrel saddle. I plan on ordering a custom Martin all around as soon as I figure out for sure whether I need the 7.5" gullet or the 8". This mare is a little tank. They have from 6.5" to 8" gullet widths and the bars open up as wide as possible to fit different horses.
I ride wide, mutton withered ponies (pretty much always have). The only saddle that works the best for both them and has the super comfort for me is my County Stabilizer XTR. It is made for me on an XW tree and that with their Logic girth has kept the saddle from moving on hills.
I know that it is not an endurance saddle, but maybe one of their other styles will be suitable with the same results.
I call my saddle my couch. The XTR leather breaks in immediately and I can ride in it all day long.
