Hi WebbyR,
The heaviest part of the body is actually the lower part of your torso not your legs or shoulders.
Post #2 here has more information about the more common symptoms that people may experience on a mattress but lower back pain is most often a sign that either the mattress is too soft and the hips/pelvis area is sinking down too far or that the mattress is too firm and they aren’t sinking down enough to fill in the gaps under the more recessed parts of your body and the spine is sagging down to the mattress from a lack of support.
If your hips/pelvis area are sinking down too far then adding a topper under your legs and shoulders could make things worse since the middle of your body would be even lower relative to the rest of your body.
If your mattress is too firm then you may need some additional support under the recessed parts of your body (the waist or small of the back).
While only your own experience can confirm whether any idea will work for you in real life … I think that adding a topper under only the lower and upper parts of your body could be risky and could make the issues worse.
There is some information about zoning in post #11 here and post #2 here that may be helpful but I’m guessing (and it’s only a guess of course) that if you are going to explore zoning that it may be more helpful either under the middle third of your body or perhaps just under the recessed part of the spine to help keep it from sagging.
Phoenix