Hi thesrod,
The main reason that manufacturers do this (use thicker layers of soft latex with lower quality/density quilting polyfoam on top of the latex) is that many people like how they feel in a showroom (mostly consumers that are attracted to buying “luxury” mattresses) and they don’t realize that the thicker layers of lower density polyfoam (more than “about an inch or so”) along with any quilting fiber will usually be a weak link in the mattress and is much more likely to soften and break down and and form the type of impressions that you are experiencing.
The support core of a mattress has a different function from the softer comfort and transition layers. There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress (or a mattress/topper combination) in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
Thre are some suggestions in post '#2 here that may be helpful with a mattress that is sagging but since the latex in your mattress is likely to still be in good condition and you spend a great deal on your mattress it may also be worth considering mattress surgery to remove the polyfoam layers that are causing the problem (see post #2 here).
The better options and possibilities I’m aware of in the Nashville area (subject to the guidelines here) are listed in post #7 here but you would need to check their websites and/or call them to find out the specifics of the mattresses they have on their showroom floor and the type of latex they have inside them.
There is more about the different ways to choose a mattress (either locally or online) that is the most suitable “match” for your specific needs and preferences and how to identify and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them in post #2 here.
There is more about the differences between blended Talalay and 100% natural Talalay in post #2 here. The biggest reason that people would choose 100% natural Talalay is because they prefer a more natural material for personal or lifestyle reasons and they don’t mind the higher cost or in some cases the lower durability that goes with it. Both types of Talalay are equally 100% Talalay latex (Talalay is the method that is used to manufacture the latex and has nothing to do with whether the latex is natural or synthetic) and there are many people who prefer an all Talalay latex mattress whether it uses blended Talalay or 100% natural Talalay.
There is also more about the differences between Dunlop and Talalay in post #7 here.
All types and blends of latex are a more durable material relative to other types of foam and as you mentioned choosing the most suitable mattress design in terms of PPP would be more important than durability issues with any all latex mattress … especially with a component latex mattress where you can replace an individual layer (usually the top layer) if it softens or breaks down before the deeper layers.
Only you can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to be able to make specific recommendations or suggest a specific mattress or combination of layers or materials based on specs (either yours or a mattress), health conditions, individual circumstances, or “theory at a distance” (see mattress firmness/comfort levels in post #2 here).
When you can’t test a mattress in person then the most reliable source of guidance is always a more detailed conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specific options they have available based on the information you provide them, any local testing you have done, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs to different body types, sleeping positions, and preferences (or even to other mattresses that they are familiar with) than anyone else.
There is more about the benefits of a thicker mattress in post #14 here but you certainly aren’t in a weight range where you would “need” 12" of latex although some people may still prefer it either because of the extra height, any differences in the “feel” of a thicker mattress, or because of the increased options to rearrange the layers and customize the support or comfort of the mattress.
Phoenix