Hi Jonathan,
If and when the time comes then post #2 here and the topper guidelines it links to has more information about choosing a topper that can help you use your sleeping experience on a mattress as a reference point to help you choose the type, thickness, and firmness for a topper that would have the best chance of success.
There are many many topper options available to āfine tuneā a mattress that is too firm including polyfoam, memory foam, latex, buckling column gel, along with natural or synthetic fibers such as wool, cotton, down, or down alternative mattress pads or toppers depending on the specific effect and feel you are looking for and your own individual preferences but I would cross that bridge if and when you get to it.
Phoenix