Fitting harness, especially one ordered by her measurements, the buckle should (ideally) fit in the center hole. This allows you to both take up or let out the straps. Animal size might be plump in spring, get more muscular with work as summer arrives.
Did you try bending the saddle to check it having a tree? Harness used with 2-wheel carts, needs a tree to keep cart weight off the pony spine. This saddle is also kind of narrow, additionally concentrating cart weight on a smaller area, while a wider saddle would have spread out the weight more.
I think your breastcollar pad has changed the fit on her. The suspender/shoulder strap is too far forward for me, making the place it crosses her neck uphill. You might try the breastcollar on without the pad, see if things move back to a more correct location on her neck. Does the seller offer a V shape breastcollar? With her low windpipe, any rise in breastcollar pushes on that windpipe. V shape might give her more clearance with a lowered head in work. Along with that, you might ask the seller if you can trade the check rein for a false martingale. The martingale will hold the breatcollar down in place as she moves and halts. It is a useful piece of equipment and useful on daily basis.
On the crupper, you actually WANT to have some “air space” between crupper and tailbone. Having it snug will sore her up in that tender location, with constant rubbing while she moves. It will also pull on the tailbone as she lowers her head, lifts her back to use her whole body on hills, deeper going, hever loads. Saddle should be straight up and down on her body as she stands still. Slack in the back strap should lay along the spine as she stands still. Again, you really want the buckle in the center hole, not the last hole, to get that fit. Thick harness pad under saddle will cause additional rocking of the saddle.
On bridle fit, her eyes should be in the center of the blinkers, to prevent her peeking above or below, seeing things she needs to ignore. Noseband should be a finger or two (skinny fingers for ponies, ha ha) below the edge of her cheekbone. Noseband goes all the way around her nose, not just a half nose strap. Full noseband keeps the cheeks of bridle against the sides of her head, again preventing peeking thru gaps of cheeks pulling away from her head. With training, she should not need a checkrein. They do not allow the equine to lower her head to use her body. Any type of snug checkrein causes hollow back, muscle development on the underside of neck. Check reins DO NOT prevent equine from kicking while hitched. Bridle should allow the checkrein to be totally removed from the bridle, get those extra parts out of your way.
And that brings us to the shaft length. A number for length is OK to have. However! Length needed can vary with the build of the vehicle and the pony. Point of shafts should end at the point of shoulder. Length behind rump will vary. She needs room to fully extend her hind legs in her big trot without hitting any part of the vehicle. We put the vehicle on the animal, then we lift a hind hoof and extend it as far back as possible. You may need a header to hold her in place to do this. Full leg extension is how much room she needs in a vehicle you hitch her to. Narrow width vehicles are more apt to get a wheel kick, startle her, ca use her to hop. Most 2-wheelers are fairly wide for stability, but the bike type tire styles can bounce easier than others, a kick to a tire might cause a bounce, then tip over very fast.
I agree with 2Dogs, those style pads are not the best choice in my eyes. They compress under weight, allow extra movement of harness even fitted tight, hold in heat during use, with both foam or felt stuffing. They look nice, but really don’t do much to aid the horse if harness fits properly.
