A small reaction in the moment is understandable. The 4 months of trauma overturning over a year of training though suggests that your horse doesn’t actually have the solid foundation you think he does. Having good ground manners when conditions are ideal is a great start, but it sounds like he could start building his ability to cope with less than ideal circumstances too.
It sounds like you’re a little overprotective and maybe walking on eggshells a bit around him, and he’s gotten used to that which makes any kind of pressure seem like a big deal to him. It’s all very well-intentioned but since he will be handled by other people he should be able to cope with normal handling practices as part of his basic training. Something like preparing him to be restrained for vet care is a necessary life skill - you personally may not want to use a twitch but he will encounter one at some point and if he responds by slamming against the wall you risk his safety and the safety of everyone around him. He sounds like a really good boy so I’m sure he’ll figure it out in no time. He just needs to reset his threshold a bit and learn to maintain those good ground manners even when things are happening that he isn’t crazy about.
and he may no associate me with the experience. 
