Hi alasearwax,
With a 3" very plush layer of 19" over a 6" 40 ILD latex core, and at a higher BMI, this would make sense (as one of the solutions I pointed out in my earlier reply) that you would desire a transition layer between your top latex and the core, as you are in essence “feeling through” the upper comfort layer to the very firm core, and while you are not compressing the upper layer completely, the softness of the compressed ILD of that layer is still quite a bit softer than the initial ILD of the support core beneath it. So it would make sense that a more substantial transition layer, let’s say 3", of a more medium latex could provide a better ability for you to “bend into” each layer and match up better, allowing more for your shoulders and hips to sink in a bit without negatively impacting alignment. The mattress then can compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights (or parts of the body).
Post #3 here has more information and suggestions about heavier weights that is worth reading.
Also, you may wish to reevaluate your pillow, as this can have a very direct impact upon shoulder comfort, making sure that your current pillow allows for a more “neutral” cervical/upper thoracic alignment.
Phoenix