Hi Jewels,
Density is single biggest factor in foam durability assessment …while memory foams have higher densities when compared with polyfoams this is because of the added chemicals that give memory foam varying degrees of viscosity… At your BMI the second layer of 2" of 1.8 lbs poly would meet the durability guidelines for a one-sided mattress 1.8lbs would more durable than 1.5 that you mention in your second construction … (I’d also keep in mind that 1.5lbs of poly is the minimum I would recommend for a two-sided mattress.) At 20 ILD the second 2" layer of 1.8 lb is primarily used as a comfort/transition layer and it will have little impact on loss of alignment. There is also 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 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” that may be useful as well as you go through this process.
You are correct that this is not ideal and I would avoid this approach because of the uncertainty involved with making the mattress that you’ll purchase fit the topper that you have. Choosing the right topper that would be suitable in terms of thickness, firmness, and PPP for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper (unless you can test the combination in person or you are purchasing both online as a “set” that is designed to work together and they both have a good return/exchange policy) and then use the option to add a topper as a “backup” strategy in case your initial choice is too firm and doesn’t turn out as well as you hoped for rather than a “primary” strategy.
Phoenix