Hi sleeplessonlatex,
How soft a layer feels is the result of both thickness and firmness which work hand in hand in combination with some other specs that can also affect how soft a layer feels as well (see post #2 here) so both would contribute to the firmness/softness along with the firmness of the latex underneath the top layers (every layer in a mattress will have some effect on the feel and response of every other layer above and below it) and the effect of the cover and quilting as well.
The specs of the PLB mattresses are here so you can use them as an approximate reference point if you decide to test them.
There is more about primary support, secondary support, and their relationship to pressure relief in post #4 here and in post #2 here but comfort/pressure relief is what most people tend to feel when they first lie on a mattress and alignment issues are what most people tend to feel when they wake up in the morning (or after several hours of sleeping on a mattress).
I would first carefully test the softer mattress (24 over 36) to see how much closer it is to your “ideal” and if it’s an improvement compared to the mattress you have now then I would strongly consider an exchange and then if your sleeping experience still indicates that it’s too firm and there are no softer options available to you then I would use the topper guidelines I linked earlier in combination with any local testing you have done that you can use as a reference point to make a “best judgement” choice for a topper. If you do end up purchasing a topper I would also factor in the “value” of the options you have to exchange or return a topper after a purchase if you are at all uncertain to reduce the risk of making a choice that doesn’t work out as well as you hoped for.
Phoenix