It sounds like you have done things fairly correctly. Most importantly spoken with your vet. And you have got a lot of good suggestions. I am going to add another to the list. It is non-invasive, and has no taste. In fact, it sounds REALLY stupid! I certainly thought it was stupid when I first saw it!!
Try putting a copper bracelet on the horse.
Learning to ride in England, there were a lot of elder statesmen in the barn. One of them was a medalist in the ’68 Olympics, in dressage, and a brilliant (if highly opinionated) 4-legged instructor. He wore a copper bracelet when not being ridden. The bracelet was a flat strip of copper, laced through a leather cuff, which buckled around his pastern.
One day there was a horrific ripple in the force: the bracelet had been lost! Within a week, the horse could barely walk. If he lay down, 6 of us had to get him back up. It took a couple weeks to have a replacement made and received. Frankly, I thought this would be the end of our excellent instructor.
Upon the receipt of the new bracelet, the horse was about 2 weeks wearing it when he bucked off his first rider. By the end of 4 weeks, not only was he 100% sound, performing at peak levels, his list of ditched riders had grown to include about 85% of his riders. Our favoured instructor was back!
I don’t believe horses are psychosomatic. I would see a great number of elder competitors – and I mean in the 20 to 30 year old age range – wearing copper bracelets. I have put copper on dogs, horses, a goat, miniature horses; nearly all have responded to greater or lesser degrees.
The man who made that first bracelet made one for me – so I would always remember that sometimes the most unheard of things have viable results!
I have worn it through nearly a decade of Illinois winters and now in mile high AZ winters. It simply works. And it is always worth a try!
Good luck 😀