Hi Bgarfield,
I think that creating a layered mattress can be a great idea … as long as you are working with a manufacturer that has the knowledge and experience to give you good guidance and has exchange or return policies that can help you if you make choices that you need to change.
Post #2 here talks more about the difference between a latex / polyfoam hybrid and an all latex mattress. I would also suggest because of your much heavier weight (which will compress more of the mattress surface) that you also consider using more than just a single layer of latex because as I mentioned you will “go through” only 3" which can add to durability issues of the layers below it. 2.17 lb Omalon is a very durable grade of polyfoam and will wear very well but it’s not as durable as firmer latex.
Phoenix