Sorry, if he was mine, I would not put the money into making him a driving pony. He has four strikes against him, he has won EVERY time there was a problem. Even Amishing him, is not going to make him reliable for YOU. You don't have the time to keep him worked regularly, and keeping him in consistant work. Most Amish horses are cooperative because they work hard DAILY. I would imagine the Amish kids will get him driving, but still may not be safe or reliable for casual use, now and again.
I would believe that you do not and can not trust him after being hurt by him in the past. Not liking a horse who scares you is fine, you are NOT required to keep and use him. Maybe you could trade even up, for another pony, even untrained, to move yours on.
Part of the driving problem could be you. You may have poor driving reactions, which was part of the problem with the originial accident. I would check around for a driving trainer who could coach YOU in rein handling, correct reaction to each kind of situation, like a spook or harness failure.
Another part of driving is having good equipment, harness and vehicle. I don't want my stuff breaking, EVER. I never recommend any vehicles with bike tires. Cheap carts, cheap harness breaks when put under stress like a spook. As you found, even little pony can be much stronger than you, when he is frightened. He has to have solid basics, be willing to work with you when frightened. Any parts of harness or cart breaking removes strength from other parts, he can't FEEL you, tries to run. You can't hold one who has lost his brain or training from fright.
Solid harness, dependable cart, do not come cheap. Many folks are unwilling to invest in good equipment, training for themselves. Driving wrecks are always bad, don't have undependable equipment to add to the problem in a tense situation.
Let this pony go. Do your homework BEFORE you go back to driving. Get lessons for yourself, get your harness and vehicle checked for safety. Maybe upgrade things before getting the next animal for driving. Set yourself up to be successful in your driving. Driving is a LOT harder to do well than most folks think. You and pony have to be partners, not just tied together with harness and reins. Keep us posted on your progress.