Hi mang0lyn,
Adding an additional soft comfort layer can certainly improve pressure relief but the effect of a topper can also be somewhat uncertain with memory foam because while a topper can add to the thickness and softness of the comfort layers and improve pressure relief … it will also affect how much of your body heat reaches the memory foam layers under the topper which can make them firmer so it can sometimes take some trial and error to get the mattress / topper combination right.
There are some topper guidelines in post #2 here and in the posts it links to that can help you choose a topper using your sleeping experience on your mattress as a guideline and reference point. It also contains a link to a post which has a list of some of the better topper sources I’m aware of.
When you have lower back issues especially … the “safest” approach would be to use the least possible thickness/softness that you believe will relieve the pressure issues on your hips because if the topper in combination with the memory foam in the upper layers of your mattress (1.2" of 7 lb memory foam and 2" of 5.3 lb memory foam) is too thick and soft for you then the risk of sleeping out of alignment and the lower back issues this can lead to would be higher.
Phoenix