Hi chloe41,
I would need much more specific information about your mattress (including the ILD’s of all the layers) along with more specific information about the your experiences and symptoms on each combination and about how your symptoms “change” between each combination. In other words I would need a reference point for the types of changes that either improve specific symptoms or make them worse relative to the other combinations even if they still aren’t “ideal” for you. Describing how the different combinations change your symptoms can help to provide a “pointer” to the types of changes that may help resolve the issues you are having.
I generally try to take a more methodical step by step approach to these types of situations so that what you “learn” from each combination can be applied to the next one.
I would probably start by listing the specifics of your first combination and your experiences and then going from there. If this combination was too firm then there are also some suggestions in post #2 here that may be helpful.
Stomach/side sleeping can be particularly challenging because they are “opposites” and the needs of side sleeping which generally needs more pressure relief and thicker/softer comfort layers are very different from the needs of stomach sleeping which generally need a firmer and thinner comfort layer to lower the risk of hyperextending the lower back by sleeping in a swayback position. In these cases “just enough” pressure relief for side sleeping and no more will generally be the least risky in terms of alignment when you sleep on your stomach.
Lower back pain is often the result of a mattress that has comfort layers that are too thick or soft and/or support layers that are too soft which can “allow” the pelvis/hips to sink down too far in one or more of your sleeping positions, tilt the pelvis, and put the spine out of alignment.
Shoulder or upper back pain can often be the result of a pillow issue or from comfort layers that are too thin or firm and don’t “allow” the shoulders to sink in enough to relieve pressure or that put the head and neck out of alignment.
At this point I don’t have enough clarity about your experiences or how they changed between each combination to be able to guess what may be happening.
Phoenix