I tried treeless (Barefoot – and loved it for myself/my own comfort) and went back to treed for the same reason – concerned about my horse’s back.  Now I have a Freeform Treeless saddle.  It has firm foam panels that velcro to the underside of the saddle that create/maintain a gullet/spine channel.  They stay put, unlike trying to stuff your saddle pad with various support that swims around under your saddle. You can widen or narrow the positioning of the panels to customize fit.  You have 3 billet options like an English saddle. The sweat marks are consistent, even under the stirrups which are very easily adjusted to each person/horse.  Even if you put a lot of weight in your stirrups while trotting. I am an average size rider ~130 lbs doing 10-35 mile rides with a fair amount of trotting. Check them out at treelesssaddle.com.  Paulita is very helpful.  I had an expensive custom endurance saddle made which was not what I had hoped for and I am still not balanced at the trot, even though I think my horse felt a significant improvement in freedom of movement.  Then I bought a FreeForm and I don’t ride in anything else!!  Lightweight, adjustable and treeless!! Yay!  Wish I had found FreeForm first!  Would have saved a lot of money.  These are not inexpensive saddles but they are worth it, in my opinion.