@aholzheu Has anyone checked you for thoracic outlet syndrome? I have TOS and in physical therapy, we stretch the tight front of my chest and neck and try to strengthen the back to keep the scapulas in place or they do wing out and the shoulder comes forward. It does cause muscular headaches on the back or side of the head. TOS is often missed and it is misunderstood by many doctors. You can find a specialist for this at a teaching medical center that lists TOS as something they treat, usually with physical therapy.

Here are some links to information.
https://www.mayoclinic.org/diseases-conditions/thoracic-outlet-syndrome/symptoms-causes/syc-20353988
https://mskneurology.com/how-truly-treat-thoracic-outlet-syndrome/