Hi djarchow,
There are too many unknowns and variables, the mattresses you listed are too different in design, and there are too many missing pieces of information to make any meaningful comparisons in terms of firmness or softness or PPP between them. It would also depend on who was comparing them because different people can feel and experience the same mattress very differently.
Having said that, as a wild guess only because they all have some missing information, and based on the top 6" of each mattress which is the part that you would “feel” the most … I would guess that the KK would be the softest, your mattress would be next and the iChoice without the topper would be the firmest. With the topper the iChoice would probably be the softest.
If you are considering building your own mattress I would make sure you read post #15 here first so you have realistic expectations of success and you are comfortable with the trial and error and possible additional costs that may be involved in a DIY design, and that a “spirit of adventure” and what you learn along the way is more important to you than any cost savings that you may or may not realize.
I don’t think you would feel much if any difference between the latex and a firm polyfoam layer of the same thickness.
All of this is as much an art as a science and there isn’t a formula that you can use to translate one mattress design into another with all the many variables involved in both the mattress and the person. There is more about “matching” one mattress to another one in post #9 here and there is more about some of the specs that can affect how different materials can feel or perform or work together in post #2 here.
Phoenix