Wonderful to hear that you see such a great change in your horse!
I have to admit that I have no experience with what I'm going to suggest neither with the kind of riding you want to achieve but maybe it will help you to think about a possible solution.
There are four things I want to suggest:
1) Reward everything the horse offers.
Therefore, I would spend a lot of time with the horse without any special goal or exercise. We can be together in the paddock or the pasture. It's an benefit if we are not in the arena because then we can do other things than doeing something together all the time. The horse can leave us and be with the other horses or go and eat some grass without getting bored because there is nothing to do in the arena. If there is nothing we can do, we could take a book or something to work on with us, so we can fill the breaks without getting bored by ourselfes.
I think it is really important not to give the horse the feeling that you are waiting for him to do something. Thereby, the horse would be under pressure again, a different kind than before, but still pressure. And in my opinion we can't push someone into being confident and brave and making own decisions. That's why I would walk around the area by myself, watching the other horses, searching for little flowers, sometimes bringing a treat to my horse, doing work or something like that. When the horse decides to join me I would reward and be very proud.
When the horse choses to be with me I would reward every little thing he does like coming to me, standing next to me, moving a step in any direction, looking at something, touching my hand, touching something else and so on.
My focus is not on the horse, so that it is not important if the horse choses to do nothing but snooze next to me. There is no pressure on the horse to do what I may have in mind because there is nothing in my mind ecxept to admire everything he does. He does not have to solve a puzzle as sometimes in clicker training. After I rewarded the horse I would focus again on what I did before.
But when I feel that the horse really wants my attention but does not know how to get it I would give it to him, still looking for every tiny thing he does on his own.
If the horse would take a step away from me I would reward and show him how happy I am about that he does not need me to tell him where he has to go. When he seems to wait for me or want me to come with him, I would follow. This can result in walking around the area together, looking here and there, doing this and that.
My hope is that the horse gets to know that he is right with telling me what he wants to do and that I am soooooo happy that I'm allowed to join him. We both experience that we can relax together and are not pushed to do specific exercises.
2) Building a language together and making sure the horse knows that a „no“ is an accepted answer.
When I think that the horse feels good and relaxed with me being around I start to make similar suggestions as the horse does. I make sure that I ask (!) all that time. That means that I reward a „No“ as well, so that we don't fall into a „right and wrong“ kind of thinking and the horse is confident to show me not only what he wants but what he does not want, too.
I try to find out which of my actions gets which response from the horse. Does he stop, move forward, away from me...? Never forget to reward everything the horse offers too. Don't forget that we want to stay relaxed and show the horse that he does not have to do something.
The Encouraging Politeness
might be very helpful to build a language together with the horse.
3) A different kind of going for a walk.
When I feel that my horse gets the idea of offering actions without me asking for it and I know how he responds to my suggestions and body language, I ask him if he would like to go for a little walk. The goal is to be as relaxed outside as we are together in the pasture. I take a long rope so the horse does not have to stay too next to me. When we are next to the gate I open it and now the horse can choose if he wants to step out or not. Both is great because it's a decision of the horse.
When the horse wants to go out I follow him and now we do the same as in the pasture. I reward for everything the horse offers. We walk around, looking at things we can find, the horse choses the direction. He can always walk back to the pasture and I will let him in to his friends. We don't have to walk far away. Every step is great and I'm really fine with it.
When I feel the necessity to ask the horse if he still responds to me I suggest little things like taking a step into my direction, walking slower or faster, stopping or just turning an ear into my direction or looking at me. I don't want to draw the attention of the horse away from the environment but neither want I to lose it completely. Therefore, I try to establish a constant but very gentle dialogue.
My hope is that the horse enjoys to be a explorer together with me and that he has fun while walking around.
4) „Stronger“ suggestions away from the horse.
When I feel that my horse has fun and is really self-reliant with showing what he wants and what not, I begin to make „stronger“ suggestions. For example when I suggest to change the direction I choose an object or an place and put my focus on it. I try to raise the feeling that I am curious about what kind of object it is or what we can explore there. The „strong“ is not directed towards the horse but away from him. I don't want the horse to go there. I want to go there for a good reason (there is lucious grass for the horse to eat, there is something we can step onto, there is something that we can destroy, there is a place we can move faster...). The thought in my mind is not „We have to go there because I want it!“ but „Wow, look there! Isn't that great! Let us go there and eat it/climb on top of it/run there...!“. It is the feeling of being enthusiastic about something.
Still the horse can follow me or chose not to. But my wish to do it is bigger than with the suggestions before. I would be careful with it when I am not sure how the horse will feel about it. Maybe it is best to do it step by step so that the horse isn't overwhelmed with it and to establish it in the pasture first were the horse is at liberty. Then we would have the benefit that we can't make the horse follow us by using pressure (with the rope) and we would have to learn to raise such a good feeling that the horse wants to go there too.
I don't have experience with the fourth point, so I don't know if it works.
Now I have written so many things that have nothing to do with riding
but I think that we can transfer it:
We already established a kind of communication where both partners can do what they want. We feel relaxed with each other around and we have fun together. We can make suggestions with different intensity. And most important: we know each other very well.
That would be the point from where I would start to do the same thing as from the ground from the back of the horse. If there is the possibility I ask another person the horse likes, to sit on him while we do point 1). I would just do it a very very short time like 30 seconds and stop before the horse feels discomfort.
Within time the horse gets to know that there is now difference between „riding“ and „groundwork“ and we can go further to step 2), 3) and 4). Chase the tiger
could also be helpful for a horse that needs the human to tell him what to do.
I don't know if this is helpful for you because I may got the problem wrong or you need a different advice. But I'm sure that you will be able to solve the problem soon and that you are on the right track!