Honestly horses weigh over a thousand pounds or 400-500kg, they are MUCH stronger than us. It is an illusion if we think we can over power a horse, especially one that knows how to get away. Im not sure that you can ever really train it out, I think you can make it better but I think it's always in there. So I dont know that it is possible to make it go completely away. They're still a prey animal and rely on instinct and impulse.
I also avoid trainers who say a horse is trained to be obedient no matter what. Absolute arrogance and total non sense. A horse is not a machine where you insert the right code and get the right output. This is not math where the right algorithm gets the right result. A horse is not a machine, there are many-many right ways of working with horses and many wrong ways but the point is the system needs to fit the horse. I find a lot of dressage trainers have "systems" and the horse either fits in their system or doesnt and often times, if a horse doesn't fit their system they pass it on and aren't willing to think outside the box or amend their system/riding to fit the horse they're working with. The system of training changes horse to horse, it isnt the same formula for every one. When I hear people make claims like that, I'm like you clearly havent worked with a large enough data set to know what youre talking about. For every person who thinks they know it all and have it all figured out, there comes a horse to show you why that isnt true.
The horse I own is a bit like what you describe but more extreme. 17h 600kg, very strong, very willful, very hot, very clever. Bit unpredictable but a good soul and very sweet. I think sometimes you have to accept the horse you're working with and accept what you can change and what you can't. I have not had a rider on him, even a friend of mine who was a GP show jumper that has not been ran off with by him. I had a while where I got ran off with probably 10 times a ride, now it still happens but rarely. I've had people tell me I'm sick in the head for owning a horse like him and people always think they know how to do it better until they actually work with him. Even very experienced, big male handlers, if he wants loose he's gone. He can drag almost anyone at the walk, non chalant, easy walk. Only horse I know who does that, you stand your ground and he uses his neck and is like well Im going this way, you can come or not
I've learned I have to have a sense of humor about it and just not put him in a position where he can do that. His nature is to push into pressure which is tricky. If I take him for walks in hand, always in a lip chain. I have tried over the nose and he just pulls loose. It is possible that you walk him in a lip chain for a while, maybe he'll stop thinking to run away? But not a guarantee.
He was a bad bolter outdoors, he stills bolts on occasion indoors but much much less. I usually think of a Buddhist temple and singing bowls when I ride him because he can go from a nice relaxed walk to extended canter in a heartbeat. I was the type who could usually take the spooky, nervous, "dangerous" horse and not have any problems hacking or out in the fields but that one was scary. Not sure how I fixed that but I waited until it was really hot and he was exhausted and just started taking him out to cool out and made it as relaxing as possible to re-train associating the field with relaxation. I also took him for walks in a lip chain and tried to encourage grass eating and relaxation. He still bolts, sometimes he jigs and can be a handful but Im not scared to take him hacking anymore. I'll take him out on the road for walks when the fields are too wet. However I almost never go above a trot and NEVER EVER gallop him in the fields or he quickly goes back to just over powering me, taking over and bolting. I can never ever let him be in control, I always have to keep him to my seat and not over power, no matter what even in the arena. I cannot let him push his shoulders out because the moment he takes his shoulders, he takes control and goes. I always have to keep him to me or he's gone. He is always a breath away from a bolt in the canter. I think it'll become easier to ride but never really go away and that your horse is more sporadic about it. I actually think it's a harder equation to solve. With mine, I just look at it as brick by brick, day to day, hope, have a sense of humor about it and accept it as is day by day and it has gotten much better.
I also have a list of priorities with him that are hard to explain but basically I prioritize safety and keeping him with my seat at all times, so there are things I'll do with him that I wouldnt do with another. I think with some questions there is only the daily work and going at it day by day. I think with a lot of things, there isnt necessarily a quick fix or a solution but a long term plan and a maybe.
I worked for a driving trainer for a while and the ground training, in hand and breaking on those horses is pretty extensive. We had a PRE that I rode dressage on and she drove that he was a bolter for a little bit then suddenly it stopped. He was very green at the time. Im not sure what we did that made the change but I think just day to day, brick by brick type work. But I rode him dressage through flag waving, electric light up toys, umbrellas, blue tarps and all kinds of things to get him to stop having that reaction. He was smart so he knew the difference. I remember we took him out in the snow for a walk and he bolted, I was bareback and just like well, at least he's too lazy to go far. Im not sure what clicked but he stopped doing it *shrugs* however I think his lazy nature was an asset.