Well, I just wanted to remind you that you've come a long way with him, and the things that you are trying are all good choices. I am not sure there is one and only one way to help a horse relax.
There is always the pain issue, so you have to be very sure that pain is not making your horse want to run out from under the saddle. I kinda doubt that, but that is the question all horsepeople must aske themselves first; is this a pain problem?
If he is nervous, he might need to move out some. Horses can really hold a lot of anxiety in their back and haunches, and the way "out" of their body is the same way; by moving and by loosening the back. easier said than done, right?
at least you can let him go if he needs to, right? and after he has trotted out vigorously, you can start to work on him stretching his head and neck further downward, which helps loosen the back and allow it to swing through the trot. All those physical movements can ease out the tension tht a nervous horse contains.
Some horses have to be worked with lots of turns, or disengagements of the hind. Those would be , I think, those that really go into a panic and want to flee.
Those that are nevous due to their surroundings I might do a lot to keep them busy, such as stopping, backing up going forward, faster, slower, serpentines, and whatever creative thing you can come up with.
Don't get discouraged. you ar doing well and have made definite improvements in your riding, based on the photos you've shared.