Hi gardenguy,
If you think you need just a bit more plushness, then your idea of adding a few more inches of plush Talalay on top could be a good solution. This would keep your “deeper support” consistent. Commonly the solution in situations like this would be to add a little more thickness to the surface (to isolate you a little more from the firmer transition layer).
If you’re going to stay within the limitations of your mattress encasement, then switching to the 23-27 ILD would be the next most logical option. With the size of your mattress, I don’t know if you have enough material to fold over your current plush Talalay upper layer to see what that might feel like on top of your polyfoam core using two 19 ILD layers.
If you end up replacing your polyfoam core with latex, you would be basically starting over with your DIY experiment, as you’d be impacting your deeper support characteristics as well as your overall levels of comfort.
While you may have run across this information previously, I’ll include it here as I know you are always interested in learning:
When you are dealing with pressure relief issues (typically numbness, tingling, limbs falling asleep, etc.) then it’s usually about the thickness and softness of the upper layers of your mattress and the “cradle” that is formed when you sink IN to the top layers.
If the more “pointy” parts of your body are bearing too much weight (what are called “bony prominences”) then you would have pressure issues because the parts of your body with more surface area would not be in firm enough contact with the mattress and would be bearing too little weight to relieve the pressure on your bony prominences.
In other words, the upper layers are about “allowing” enough sinking in to create the mattress cradle which is a larger area of surface contact with the mattress to relieve pressure and support the recessed areas of the spine. The deeper layers are about controlling any further compression than necessary for pressure relief to “hold up” the heavier parts of the body and keep the spine in its range of natural alignment. The amount of sinking allowed by lower layers has less effect on pressure relief in other words (in most types of mattress construction) but controls alignment. This is why when people change the firmness of upper layers to solve support issues or the lower layers to solve pressure relief issues they will often create new issues and may not solve the issue they are trying to “fix”. This is also why describing a mattress as a whole as either soft or firm can be very misleading because all mattresses need some of both. There is some much more detailed information on shoulder and arm issues in posts #2 and #3 here.
Phoenix