OH, I love your horse! What a great pair!!!
Your position: I agree with others about your leg and heel.your lower leg and heel move up and down when you trot. That happens with many people you can improve your leg by thinking about pointing forward with your knees (which are a tad rolled out) and toes. This will help put your leg in a better position and your posting won’t make your heel hit your horse every stride. That will kind of dull him to the aids.
Next, I’d suggest thinking about posting using your hips to swing forward from your knees (without gripping with your knees). This will make your lower leg action (or no action) independent from your posting. Same with your shoulders, try to make them still and independent of your hips moving forward when you post.
Arms: try to bring your elbows closer to your body and keep your hands a bit closer. Try to really follow his movement with your arms at the canter and walk especially. Going to the left looked more fluid in your elbows than going to the right.
Hands: keep the thumbs up like you are holding a mug of water.
Seat: really think about letting your seat move with the motion of your horse in the trot and canter. For example, look at your transition to walk and turn on the diagonal. Your arms are moving but your hips aren’t following at all. Relax a bit in your lower back, let your shoulders come back and let your hips flow with the horse independent of your legs and shoulders. This’ll all improve your transitions.
Are you coming from H/J land?
This is a long list - I’m sorry! It’s all fresh in my mind because I think about this stuff every time I ride! I have a wonky right arm that I’m always called out on. I’ve had other position issues (esp coming from H/J and then eventing land long ago) that I had to really work on. It is a journey, not just a quick fix! And look how well you’ve done with your pony! He was almost totally square on your first halt!
You both are doing so well!