Hi LAH7,
Unfortunately there are as many variables with a topper as there is with a mattress and the only reliable way to know which mattress/topper combination will work well for you is your own personal experience. A topper is a good option if you need extra pressure relief but it’s not a good way to improve support. Based on this comment …
It almost sounds like your comfort layer is already too thick/soft in which case a topper may not help much (and could even make things worse in some cases). You can make a mattress softer with a topper but it’s not a particularly effective way to make a mattress firmer.
I’m also not clear on how your shoulders could be sinking in too far with some of the toppers you mentioned because if you are a back sleeper then your shoulders aren’t in contact with the mattress and wouldn’t really be sinking in at all (shoulders sink in when you sleep on your side).
If you need some extra softness and pressure relief there are some guidelines in post #4 here that are based on your assessment of the amount of extra softness you may need that may help but I’m not clear that this is what you need.
Overall just not sure what you are looking to accomplish or what your symptoms are when you sleep on the mattress by itself (with no topper).
It would be helpful if you could let me know more about the specific symptoms you experience when you sleep on the mattress by itself (without a topper) and the areas you experience them. The first step in deciding on a topper is to have clarity about exactly what you want to accomplish and from your descriptions I’m not clear on the “symptoms” you are looking to “fix” or whether your issues are about pressure relief or alignment.
Phoenix