Hi D9401,
It’s great to see you are making good use of the information here 
I’m looking forward to finding out what you end up deciding … and of course any comments or questions I can help with along the way.
It may also be worth considering adding a topper to your Stearns & Foster mattress. While there may be some trial and error involved in choosing a suitable topper … if you do choose a suitable topper then it can also have the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (the upper layers of a sleeping system will tend to soften and break down before the layers underneath them) or if your needs or preferences change over time and a topper can also help extend the useful life of a mattress underneath it.
If your mattress is still in good condition and there are no soft spots or sagging in the mattress and if the only issue with your mattress is that it’s too firm and you just need some extra softness and pressure relief then a topper can certainly be an effective solution and there is more information about choosing a topper in post #2 here and the topper guidelines it links to 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.
Phoenix