Hi Ari,
Some of the common IFD/ILD ranges for polyfoam are listed inthis chart from the Polyurethane Foam Association. While you of course want to know all of the layers of whatever mattress you’re considering, as all of the layers of a mattress work together, a 30 ILD would be on the generally accepted “lower” end of the spectrum for IFD/ILD of a polyfoam core for those of a higher BMI. The density of the foam at 2.25 lb would be good quality.
The Aerus memory foam at 5 lb would be a good quality memory foam, but memory foam itself is not a “supportive” material, but instead relies upon the firmer and more resilient foam layers and components placed beneath it to provide deep support and assist with maintaining a more neutral alignment.
A transition layer could be latex, microcoil, natural fibers, or buckling column gel (I wouldn’t recommend more memory foam at a higher BMI), but most common designs would probably be a layer of polyfoam. I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or alternatively use a “bottom up” approach (see post #2 here).
When designing and building your own DIY mattress out of separate components, the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process.
Have you contacted Rocky Mountain Mattress? They are a site member here and have configurable memory foam options using Aerus memory foam plus high density polyfoam cores of higher ILDs. They are quite experienced with different memory foam designs and may be a good reference for you as well.
Phoenix