Hello JayJay
I am sorry that your significant other has back pains and I understand why you’d be in a rush to find a solution. From what you describe it seems that both the support and comfort of your current mattress are not appropriate but I’d be a bit careful with the sense of urgency that you are placing on buying a new mattress without really understanding your needs and preferences. I’d first investigate a bit in the cause of your pains. I’d look at the impressions of your current mattress and measure them. I’d look at the foundation and make sure that it is flat and even when someone is on the mattress. About the pains that your wife is experiencing you did not mention your sleeping positions but any shoulder pressure points and any pain can come from a mattress that is too firm and puts direct pressure on the shoulders, the shoulder blades, and cause soreness or numbness and in the arms or can often come from lack of plushness or postural issues as well. Lower back pains generally come from a mattress that broke down and created impressions or that has upper layers that are too thick and soft and/or support layers which are too soft which can allow the pelvis to sink down too far relative to the rest of the body. A mattress that is too soft can also allow the hips to sink down too far which flexes the hip joints out of their neutral position which can also lead to joint soreness or back soreness if the muscles are working to keep the back and hips in alignment. You can find a bit more information about sleeping positions and posture this in this article here and it is important to understand where the pains are coming from so that you can address the root cause.
Buying a mattress is a more organic process and if you try to rush it you can do more harm than good. A DIY project as well is a bit of trial and error process and unless you are closely guided by someone with the great deal of experience in the type of mattress and materials you are interested you’d be better off just purchasing a component style system where you can swap out and exchange layers to adjust comfort and support as needed for both you and your partner.
A 10" to 12" mattress thickness (as the one you described) would have the benefit of being a little more adaptable for higher weights and multiple sleeping positions, but to guide you I’d need to know a few more things, like … what is your budget range? Do you or your partner have issues sleeping hot? Do you have motion transfer issues? If you want to stick with the DIY that you listed above what is the ILD of the 5lbs memory foam you were considering? Is there a reason you are choosing a Memory Foam comfort layer? Have you tried latex as the top comfort layer and did you like it? (Latex contours just as well and cradles your body but has less issues when it comes to motion transfer and temperature regulations) What is your and your partner’s primary sleeping position?
While your proposed DIY can work well for some people I cannot speak of how this will feel to you as it is dependent on too many variables that are subjective in nature and only you can tell when you test the combination, but I’ll make a few comments more general in nature.
You did not mention the density of the 7-8" of HD polyfoam but generally 2 lb density would be appropriate for your weight. (At 1.8 lbs you can see early breakdown of the foam for higher weight people)
1" of medium-firm latex is a good quality and durable foam
Also you did not mention the ILD" of the 2" of 5 lbs Memory Foam but a firmer version of it would be appropriate for your weight range although being in the top layer for a high weight person I’d expect some break down over time.
I’d also make sure that you visit the Mattress Shopping tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices … and will help you understand how to assess your needs and preferences. Also, the links it lists would give you basic steps in looking for a suitable mattress.
After you read the few articles I referenced, you will have a better understanding of how all these variables are interrelated; please note that there is no simple answer that replaces your own due diligence and research.
LMF