Hi rob2121,
I wasn’t aware that they have the option of customizing the firmness of the polyfoam base layer in the Alexis.
In any case … they will be the best source of help and guidance about the options they offer that have the highest chance of meeting your criteria both for comfort and durability. The more information you are able to provide them on the phone about previous mattresses you have slept on and liked, your local testing, and information about your body type, sleeping positions, and preferences … the more you will help them to help you make the choice that you feel best about. They will know more about the feel and performance of the materials they have available and how they interact together and with different people than anyone else (see mattress firmness/comfort levels in post #2 here).
The comfort layers you are suggesting would be quite soft “on average” for your body type.
If you are buying a king size … it may also be worth considering using a different firmness on each side.
Phoenix