Did you inject the SI or just the neck? Sorry if I missed it. Despite the shifting front limb lameness, I would go there next. I have plenty of SI sagas discussed on this board. Had one mystery case horse who had a lot more wrong than his SI, but the SI was a definite thing despite normal ultrasounds and bone scans (ultimately this was confirmed on necropsy).
One of my current horses will have front limb stuff (though in my view, it doesn’t block…vets occasionally disagree, and to me I think there is some unintentional bias that can happen during those tests), and he even got to the point I thought he was neuro because he would trip badly in front and even went to his knees on some occasions. But it was primarily the SI (his was confirmed via ultrasound luckily being in a spot that could be seen) keeping him from carrying weight behind properly and somewhat complicated by other issues (like a couple of fused lumbar) and asymmetries that would have kept us spiraling in diagnostics had the one ultrasound not been helpful.