Hi mom2meredeth,
A medium firm comfort layer in the range of 29 - 31 ILD would be on the firm side for most people of your body type so I can understand that you are having some shoulder pain.
The risk of adding a topper is that the softer and thicker it is … the further away from the support layers you will be and this can increase the risk of exchanging a pressure relief issue for an alignment issue (your pelvic girdle could sink down too far and tilt causing your spine to be out of its natural alignment). Because of this … it’s usually best to use a topper that is “just enough” in terms of thickness and softness so that the risk to your alignment is minimized.
While only your own personal experience on a mattress/topper combination can know for certain how well it works … there are some topper guidelines in post #2 here (and the posts it links to) that may be helpful in choosing the best thickness and softness.
I would probably lean more towards 2" just to reduce the alignment risk
Phoenix