Hi JannyGoats,
My general “rule” with topper would be to use “just enough” thickness and firmness that you believe would relieve pressure because this is generally the least risky approach for alignment. In the end though … your own personal experience and trial and error is the only way to know. Slightly thinner with the same ILD would put you closer to the firmer support layers and would “stop” the heavier parts of your body from sinking in as far but would still “allow” your wider lighter shoulders to sink in more easily until they “reach” the firmer layers. Slightly firmer (with the same thickness) would create a slightly firmer sleeping surface which would also reduce the amount your hips/pelvis are sinking in but may not “allow” your shoulders to sink in enough. My “order of priority” for a topper would be good alignment first (without any symptoms of pain or discomfort), good pressure relief second (again without any symptoms of pain or discomfort), and then finally the “feel” or what it “feels like” (as long as there are no symptoms of pain or discomfort) which is much more subjective and can often just be part of getting used to a different material or sleeping surface.
You can read some thoughts about convoluted layers in post #2 here. They will be softer than the same ILD and thickness in a solid material but will get firmer faster as you sink into the material more than a solid layer that is the same type of material and the same thickness in a softer ILD (that would be equivalent to the “effective” ILD of the convoluted topper). As with any topper … the only way to know with any certainty how it will work for you is based on your own personal sleeping experience.
Phoenix