Well, I will ignore the kicking when asked to speed up since you seemed to work through that. Being herd leader would jive with wanting to be first on the trails in my mind and perhaps in his. Perhaps he does not see the trail experience as a work effort but some sort of weird turnout where he has to take you along.
How does he behave if you take him out alone?
I have had horses like that – the one I am thinking of was a very mentally competitive and dominant TB. He had to lead. If he did the experience was not bad. If I tried to make him lag or stay in the middle it was not a very pleasant ride.
If he is more relaxed out alone, that might confirm the competitive issue. You would not have to go far to test this out I don’t think.
What about with another single horse? Same behavior?
If it is his personality, I doubt you are ever going to change it materially so I suggest you start letting him lead and try to get him relaxed so that he can be next to another horse. If it has been 5 years, the behavior is ingrained and I suspect may be modified slightly but may never get to the point you want where you can totally relax at the back of the pack.
How is he in ring work with other horses? If this does not show up, then you somehow need to connect the tolerance of not leading in the ring with not leading on the trail.
One thing that did temper my guy over time was constant circling when he would get out too far. He was great alone… brave, fearless, quiet. But with others he had to be first and did not understand why I would not always let him be so.