Yeah, wide and flat pretty much means that the tree can’t make good use of the withers if the saddle starts to go sideways.
A good, knowledgeable friend of mine who has made lots of saddles and ridden more horses than most people will ever see, tells me that a back like that pretty much can’t be properly fitted. You can get close, and ‘close enough’, but it won’t ever fit just right, like a horse that has withers.
Combine that with a horse that likes to drop his back/bend his ribs in one particular direction, and the saddle will pretty much go over to the side no matter what you do-until you can teach the horse not to dump the weight of the rider over like that. And the rider has good enough balance not to have more weight on one side of the horse.
So you’re stuck getting the cinch really tight, which the horse won’t like, and using tacky saddle pads, too.
Great advice above, to use a roller cinch and to tighten a little bit at a time, a little more, a little more, as many times as needed. Some horses will let loose of the cinch-grumpies if you are always careful. I have one previously cinchy mare that I retrained with cookies- a cookie every time I tightened, worked down to two or three cookies for a six-tighten session, and now sometimes I might have a cookie but sometimes I might just scratch her favorite spot.
I have two really neato QH mares, but it is a nightmare with their barrel backs and lack of withers to keep a saddle still unless the rider is very balanced. One mare was really, really bad about dropping her back, but now a good rider can keep her using herself properly, so it’s better. I still don’t want to go roping anything with her.
But they are by far the most comfortable ones to ride bareback!