Hi coast,
The first thing I would suggest is a more detailed phone conversation with the manufacturer of your mattress so you can “talk through” what you are experiencing and the types of changes you would like to make. This will generally be more helpful and reliable than trying to describe your more subjective experiences on a forum. It can be very difficult to accurately describe your experiences in writing. They will also have more experience helping customers that are similar to you to fine tune their mattresses than anyone else and they do this every day.
It would also be helpful if you could describe your experiences in a little more detail especially whether you are looking to change the comfort/pressure relief, the support/alignment, or just the “feel” of the mattress and the specific “symptoms” you are experiencing (pain, numbness, discomfort,), where you are experiencing any symptoms, which sleeping positions you tend to experience them, and whether they happen when you go to sleep at night or when you wake up in the morning.
Some of the possibilities that may be worth considering (after a more detailed phone conversation) include trying M/S/F which would give you a firmer sleeping surface where you sink in less but would still have some of the additional softness of the soft layer in the middle or S/F/M which may feel a little softer than the S/F/F although some people may not feel a lot of difference with changing only the bottom layer.
If you find a combination that is “almost there” but is “just a touch to a little” too firm then a thin topper may be just enough fine tuning to give you the extra softness you need and help isolate you a little more from the firmness of the firm layers underneath. If you do decide to go in this direction after some experimentation with different combinations then post #2 here and the topper guidelines it links to can help you choose the type, thickness, and softness of a topper that may work best for you based on using your sleeping experiences on a specific combination as a guideline.
Phoenix