Hi seldel.
Sorry to hear about the shoulder pain.
Phoenix has some words about pain from/on a mattress in this post:
Shoulder and arm issues can come from a mattress that is too firm and puts direct pressure on the shoulders, the shoulder blades, or on the back muscles and can also cause soreness or numbness and tingling in the arms or can often come from postural issues as well. If the mattress is too firm then if you sleep on your side your shoulders may not sink in enough to relieve pressure and your upper body can “twist” away from the pressure so your upper body is “twisted” more forward while the lower body is still on its side. This spinal twisting or torsion can twist the spine in the upper body and lead to soreness in the area of the twist. There is some much more detailed information on shoulder and arm issues in posts #2 and #3 here.
Something to consider would be your pillow. People often don’t update their pillow situation when they replace a mattress.
To quote Phoenix (again):
The pillow that works best for you can vary with different mattresses depending on how much you are sinking into the mattress and the distance between your head and neck and the mattress surface. Softer mattresses may need thinner pillows and firmer mattresses may need thicker pillows but in very general terms side sleepers tend to need higher profile and possibly firmer pillows. If you are a side sleeper and your pillow is too thin or too soft then your head can bend sideways towards the mattress which could impair blood circulation or pinch nerves and could also increase the pressure on your shoulders. One indication of a pillow that is too thin could be if you sleep with your hand under the pillow.
Is a memory foam topper atop the rest of your latex build out of the question? I know we tend to sound like latex foam purists around here, but at the end of the day your personal preference and support needs > what we think or say.
A lot of people find the memory foam + latex combo to be really successful.
The other option could be to switch to a soft Talalay instead of a medium before opting for memory foam.
You could also try swapping the dunlop layer you currently have on top for the talalay layer underneath and see how that helps.
Please do keep us posted!
NikkiTMU