Hi chudd.
Welcome to our Mattress Forum! 
I am sorry that you and your husband are experiencing pains but the good news is that you’ve collected enough data points and that you are looking in the right direction with your research and local testing.
While Zotto (one of the “simplified choice” mattresses available on the market) does not have lower quality materials in their mattress relative to your BMI, it looks like their somewhat unusual design using 6" of specialty foams is too plush for you and does not offer the primary deep down support that you need for posture and alignment. Zotto’s suggestion to add a topper to fix the lack of deep support won’t work because “support” primarily comes from a combination of the firmness of the deeper layers and the thickness/thinness of the upper layers (how far away you are from the deeper support layer) and not nearly as much from the firmness of the topper, while pressure relief comes primarily from the softness and thickness of the comfort layers. This is why the comfort layers need to be “just soft and thick enough” to provide good pressure relief in the most “pressure prone” sleeping position (usually the side for those who sleep in this position), but more than “just enough” can put you too far away from the support layers and not “stop” your pelvic girdle from sinking down far enough. It’s not really possible (except to a limited extent) to improve the deep support of a mattress where the upper layers are too thick and soft or the support layers are too soft by adding a firmer topper which will “bend” into any softer foam below it. This “bending” will happen more if the topper is firmer than the layers below it (what I call a dominating layer) because the lower layers will compress more than the upper layers and the topper will “bend” into the compression of the foam below it. You will also have a sleeping surface which will be slightly less conforming and feel firmer (less pressure relieving) with a more “on the mattress” feel than if you were sinking into a softer topper even though the next layer down is still compressing. The real solution, in this case, would would have been to remove or replace softer support layers with firmer ones (which can’t be changed with a topper).
As you most likely are aware Luma (closed business as of 2023) is one of our our Trusted Members here which means that I think very highly of them their products and the advice they provide. Stephano is very skilled at offering great guidance based on customer experience with other mattresses, somatotype, and BMI. The Hybrid Latex Slumber System (14" thickness) model that you are considering is using Talalay latex for upper layers, which will be more supportive/buoyant than the Memory Foam you’ve experienced with Zotto. The good news with them is that you can customize your mattress both before and after purchase at no additional cost. They have an interchangeable comfort layer system that allows for changing the firmness feel to get the desired comfort and appropriate support. Click here for ]Hybrid Slumber System Mattress Description & Specifications.
I am not sure what Fox has at the moment on their floor but they certainly have traditional innerspring with heavy duty continuous coils, which have a different feel than the pocket coils used in Luma Mattresses you are looking at. Fox Mattress makes custom innerspring-mattresses using pocket coil as well, but you would need to call and ask if they have one available for you to test. As you know Fox Mattress is one of our Trusted Members here and whether you buy from them or not part of their mission is to cut through the marketing stories and help consumers make an informed and suitable purchase.
Luma and Zotto are different matress categories and it is not possible for me to say which of the two options (1.5" or a 1.5"+3") is best for your particular needs. Based on your description and hte right appropriate firmnes selection for the upper layers they could both work . While I can help with is “how” to choose it’s not possible to make specific suggestions, not only because the mattress type and designs you’ve tried are far apart, but mainly because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about what would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here ). This being said you are in good hands with Luma as they would be the best to discuss with you based on the information you provide about yourself and the history on different mattresses and to make suggestions based on the “averages” of other customers that may be similar to you. Of course, the option that you have with them to exchange layers or the mattress itself or use other forms of fine-tuning after your purchase or the return policy may also be an important part of your personal value equation n to offset the risk that can go with any online purchase.
If you have other questions, please feel free to post back and I or any other Expert Members of the site would our best to answer them.
Phoenix