Those mattresses are designed to protect skin, not joints.
If your skin is doing well and is not at high risk, would consider a medical grade foam mattress and a latex or other foam overlay, and a latex or foam pillow. There is a good shoulder pillow on Amazon.
You also want padding under the elbows at all times -- we used combinations of travel pillows and foam blocks but if you are not supine all night, you could just attach padding that moves with you, like an elbow cup. The shoulder joints get iffy without muscles to keep them in place.
And if your legs are splaying, consider a foam-lined belt around the upper leg, and/or pressure boots tied together with a sash. The idea's to keep everything in line to avoid straining the hips, which affects the torso all the way up.