Posting a link to a scientific researcher that actually seems to really understand TOS.

How to truly identify and treat thoracic outlet syndrome (TOS) - Treningogrehab.no

Take notice, tight muscles are weak muscles. Stretching and massaging constantly will likely do nothing in the long term and can be bad without strengthening.

Also "down and back" shoulder position is very bad and he provides good reasoning for that.

Worth a read for anyone looking through here and trying to understand.

I hope the best for everyone going through this and that you may find your answers. I like to share any information I find.

Best.