My mare also has some cervical spine lesions, so that is worth looking into, as well. If you have a good massage therapist available, that can help both in helping your horse feel better muscularly and in possibly pinpointing areas to look into. A recurring issue at the origin of the splenius muscle on one side of my mare's neck is what spurred my vet to suggest the cervical x-ray series.
A note on soft tissue: A core lesion may not show swelling; a stoic horse may not show pain on palpation or flexions. This I learned the hard way! Now, if there is the slightest possible suspicion of a tendon/ligament injury, I get the sharpest lameness vet I know out with his/her u/s.
OP, best of luck to you and your fine old fellow!