Hi sewcrazy64,
I don’t have any specific suggestions because unfortunately there isn’t a formula that can predict how well any topper will work for a particular person on a particular mattress with any certainty because there are too many unknowns and variables involved to make specific topper suggestions and because softness/firmness can be so subjective and relative to different body types, sleeping positions, and individual perceptions as well as the firmness/softness of your mattress which will also affect how soft or firm your mattress/topper combination will feel as well. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here).
Having said that … there is more information about how to choose a topper in post #2 here and the topper guidelines it links to about firmness and thickness that can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of as well (including Flexus)
Outside of the firmness and thickness suggestions in the topper guidelines … when you can’t test a particular mattress/topper combination in person then the most reliable source of guidance is always a more detailed conversation with a knowledgeable and experienced retailer or manufacturer such as Flexus that has your best interests at heart and who can help “talk you through” the specific options they have available based on the information you provide them about you and your mattress, any local testing you have done, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific toppers to different body types, sleeping positions, and preferences than anyone else.
Phoenix