@J-Lu, I think you have read the gamut of ideas and experiences from people with horses like your gelding. I’ll probably raise some hackles with my suggestions, but here goes.
Another person on this thread mentioned methocarbamol, a muscle relaxant. I found out it is used regularly by BNTs in Florida. I’m not sure how you feel about using something like that, but I think you are at the point where you may need to say “what the heck.”
I mentioned my hyper-vigilant, spooky gelding. I started him on MagRestore years ago and it did take off the edge. As a daily supplement it is good, however, I used Nupafeed magnesium paste last year when showing my mare. She is more hot than spooky and found the warm-up arena at shows overwhelming. My trainer gives it to all of her horses at shows and I decided it was worth a try. It made a huge difference in my mare’s behavior. She was very relaxed and more focused instead of tense and strong. Maybe you could give that a try to see if it will help ease his tension.
Also, I do believe when horses are constantly tense, they can produce more stomach acid. I think another person mentioned this, too. I had a bag of Daily Gold Stress Relief in my garage (I won it at a show), so I thought why not use it and started feeding it to my gelding. It’s a clay product that acts as an acid buffer. He is a fence walker and very herd bound. His behavior did change! I even did an experiment and stopped feeding it for a few days. The behavior returned, so I added it back to his diet and his behavior mellowed.I’m a skeptic who will “believe it when I see it,” and I believe it works for my gelding. Under saddle he has been much better, too.
Like you, I have been tearing my hair out over my gelding’s behavior for many years. I bought him when he was three and he’s always been wired differently. Super easy for me to start and train, but very spooky. I have owned and shown lots of horses and only once had another one like him. He was Lipizzan/TB cross. I sold that horse to a lower level rider who put less pressure on him, but he was still quirky.
Your horse now has an ingrained pattern of behavior. Whether you use a supplement or some other type of behavior modification, you have to break the pattern. Again, I’m only writing about what worked with my gelding. Believe me, I share your frustration of having a beautiful, talented horse who is an incredible challenge.