Congrats, looks like a pretty good harness.
On the breast collar, your traces go from the horse side through the cut out, then to the buckle. It is like an extra "keeper" to help ensure the traces do not come unbuckled.
As far as the other end of the traces, the cart side, I can not see the photo very well of your cart. Do you have a singletree? That is a very important part of the cart, without it the pony's shoulders will become very sore.
Can you take a better photo of the front of your cart, where you attach the traces to it?
Again, congratulations, can't wait to start reading and seeing your progress in driving.
The best time to plant a tree was 20 years ago, the second best time is today.