TWH, generally take a Gaited Saddle, there are basically two sizes of Gaited Saddles, so it is a matter of finding the correct one.
Flex Tree Saddles do not work for Fitting hard to fit Horses, they flex from a front to rear cross quadrant for a little extra comfort only.
Treeless Saddle IMO are a gimmick, a Tree of some sort has been used in English, Western, Australian Saddles and any other Variation of a Saddle from the start of time. I know many love their Treeless, no disrespect meant, but I just do not see one of those Saddles lasting long enough to be handed down from one generation to another......
From what I hear the only Good Treeless Saddle cost over 1,500.00, a decent semi-custom western saddle is about the same maybe a little more and would hold its resale a lot better. The Treeless like Hilason and some others under 800.00 have very bad ratings and are made in India with inferior components
If you have one of the Wider TWH, there are a few Makers like Dakota and Bighorn who have a TWH Branded Saddle that you may be able to find used for 500.00 to 700.00, if that is your budget.
The Narrower TWH models are a little harder to find used, but are out there.
May all your Trails be happy and safe ones