I've had this recurrent shoulder issue that has been plaguing me for some time. I know it's based on how I sit at my desk and so on. I finally searched it up and lo and behold it's literally called "mouse shoulder". I had switched back to using my full size keyboard with numpad (because I love the numpad), but I think that's a huge part of the problem.
I already have a vertical mouse and all the other things (kneeling chair, standing desk, monitor stands). I decided to get yet another mini keyboard and a ball mouse to see if that will be the last nail in the coffin for this problem.
I did find a cool tip -- put your lacrosse ball or tennis ball in a sock to make it easier to massage under the shoulder blade. No more standing in weird positions against the wall!
I also picked up a pair of Delta Hub Carpio G2.0 wrist wrests. They take a little getting used to, but overall I'm happy. My brother is printing a mod to tilt my dominant hand for the angled mouse.
