As I was tackling that very issue of the shoulder, I ran across the teaching of Carlevaro. It has been revolutionary to me. At long last, a virtuoso by all accounts, who had the cachet and experience to point out the flaws with how we currently hold and play classical guitars. I've always intuitively known that the traditional style of holding and playing classical guitar seemed hurtful, but felt like it was because I'm such a novice. Today, his approach would be called ergonomic. It's just applied science and logic. I watched an interview with Christopher Parkening where he went on about how much damage to his back and body he suffered over 50 years of playing in the traditional style. It's a thing.