Insight:
Your horse’s way of moving is less than ideal. What’s interesting is how the happiness/harmony in general may be affecting his way of moving… or vice versa!
Sometimes a change in the horse’s state of mind due to the quality of his life or your relationship with him can really change his posture. That makes sense, right? When we are happy and feeling healthy we carry ourselves really differently than if we are feeling bored or unable to make our own choices.

On the other hand, if we feel too much discomfort or tension whenever we exercise it can leave us feeling sore, grumpy, and mis-trusting about our personal trainer, and we aren’t so happy to go to the gym! This can happen with our horses too.

My Best Advice:
I recommend you address both the foundational happiness and the biomechanics. Since your communication skills are already good I know you will be able to do this. You will want to stay very aware of the layers of progress, and how an improvement in one area can positively affect the other. It will be so interesting!

Recommendation:
The BEST Dressage Naturally resource for you is the Video Classroom.
The Classroom has a wide range of videos on everything from partnership, to biomechanics, to gymnastic development, to mindset. All the videos will have a consistent theme of improving the happiness and harmony between you and your horse on the ground, on line, and riding.