I had an 275 medium ( which has the wish bone ) , but I found I didn’t have enough stand over, so I went for the gen 5 in small. I can’t feel any difference in sizing with the same components, except I can stand over it comfortably. I think Cy told me that the small was now almost as long as the 275 medium. I think you will be impressed, my gen 5 climbs so much better with a 140 fork, in fact I will say climbing is now enjoyable.
i put a 35 mm stem on mine because I found the medium too long, and it is fine on the small.
I have dodgy shoulders too, have you tried a variety of rotator cuff and shoulder girdle exercises? I recommend as many different exercises as you can, so as to not repeat them for a few weeks.
What will you fit it out with?