Hi Pspa123,
There are many part of “mattress theory and design” that are counterintuitive. Some of the firmest mattresses on the market have very soft layers of foam on top that are relatively thin to provide a softer surface feel but quickly get firmer as you sink in deeper and begin to interact with the firmer layers below.
I’m not sure how long you’ve had your mattress and topper but some of the suggestions in post #2 here may also be helpful before you consider a topper because the design of your mattress would be suitable for her based on “averages” (although any individual can easily be outside of any averages that would work for other people with a similar body type and sleeping style) and she is probably quite close to the layering that would work well.
In some cases a pillow that is too thin for side sleeping can also contribute to shoulder pressure.
Phoenix