It’s not the dressage saddle that causes the issue. The saddle I trail ride in is a flat-seat XC/jumping saddle.
Figured out this weekend that it’s not an issue on my gelding. I rode him a week and a half ago in my dressage saddle, mostly walking, and was fine. Rode him last weekend bareback, and was fine (well, hips were sore, but back was fine).
Of course, my mare brings her hind legs much more underneath her, and she’s built straighter behind. Her walk is just more jarring in general. So I’m thinking that’s the issue.
And I do have and use a Mattes, with shims in the front, to help lift the saddle a bit. She’s perfectly happy with the shims. The saddles have both been checked by a professional, actually the one who designed my dressage saddle, and were given the OK.
As for the bucking strap issue, I don’t think reins are going to save me if a deer ran into my horse. The mare is not spooky, and she’s the only one I ride like that. And trust me when I say, she will stop from my seat if she’s in the mood, but if she’s not in the mood, the reins aren’t going to do a whole heck of a lot. A lot of the time I trail ride her in a rope halter anyway because she is so good. Her brain only leaks out her ears when she’s asked to work in the ring. Otherwise you could put a monkey on her and she would be just fine.