It is hard to see, I would like to see a closer photo. I like the breeching height as it is, but if Grace stands square it might be a tad too low, but it looks good so far.
The saddle is where I find it hard to see. It looks a bit too far back, BUT if your pony has a long sloping whither it is correct, I cannot tell, even zooming in. You don't want the saddle sitting on the whithers. Especially a saddle without a tree. If your saddle doesn't have a tree I would strongly recommend a pad. Does you harness have two girths? One to hold the saddle on and one to hold the shaft loops down. If it has two girths they should lay one on top of another. Your holdback straps are connected to the shaft loops so that might be pulling the outer girth back. You want to have the saddle as vertical as possible, but often times that is not possible because of horse size or confirmation.
She looks really cute in blue!!!
I would love to see a closer photo.
The crupper should just touch the underside of his tail. I cannot tell if it does, or if it hangs down. I think you have room to shorten it on the turn back.
You may want to attach the buckle end of the holdback to the cart, and leave it there, then you just clip that snap(now clipped to the girth) into the front ring on the breeching. It just makes hooking up a little quicker, because sometimes the holdbacks require several wraps, and there is an art to doing it correctly.
My horse likes a padding on the saddle area with his nylon harness. He also has a portion of harness that goes around the front of his chest too. It may be just a different type of harness as you can have a collar as well. Although, we do use a collar for heavier type pulling for my mare in the wintertime for sleigh riding .