I ride my mare in a treeless Bandos saddle. I love it. I’d tried at least ten different saddles, and nothing fit her – either a bad sweat pattern after a ride, or white hairs down the road. A pad specifically made for treeless saddles is an absolute must, because that is what distributes the rider’s weight. Riding treeless does feel different, and I think you’ll either love it or hate it. My mare is a TB with pretty good withers, a short back, and large shoulder muscles.
These do come up for sale secondhand, I have seen them for sale with the Barefoot seat which some people feel is a bit softer than the original.