Hi Slobjones,
Thanks for the clarification. Sorry to hear that the sciatica is triggered by your current mattress and that you have to do pillow permutations.
I don’t think it is a matter of foam mattresses “being a bad idea” … it is more likely the construction, the type of foam, and softness firmness that is a good match for your PPP (posture and alignment, pressure relief, and personal preferences)
Aside from using pillows under your trunk is there anything else you tried? What type of foundation do you have under your mattress? I always suggest a ground-up" assessment to make sure that there is nothing under the mattress that may be contributing.
If the only issue with a mattress is that it is too firm and if there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the mattress/topper combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
You may also want to reassess both primary or “deep” support and secondary or “surface” support and their relationship to the firmness and pressure relief and the “roles” of different layers in a mattress (see post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”. Post #6 here has some good information as well.
Phoenix