Something to consider is Verhan, either the Futurity or the Todd Minikus. They have a wide channel and look to accommodate a long wither. I’m trialing an older Todd Minikus in buffalo and thinking it is a keeper for my Arab-warmblood cross who looks a lot like the thoroughbreds of my long ago youth.
There is a screaming deal here you might want to check out. The leather color is the older London type, but it should darken with oil. This saddle is wool flocked and can be adjusted.