Hi 01svtL,
This is the key behind “just enough” and no more when it comes to the thickness and softness of the comfort layers (which includes the toppers and the top layers of the mattress). If you have just enough softness/thickness in the comfort layers in your most pressure prone position (usually side sleeping) then there is much less risk of alignment issues in your other sleeping positions. The thickness and softness of the top layers work together to create the pressure relief you need and also to provide secondary support which “fills in the gaps” in your body profile to help maintain the alignment of the more recessed parts of your spine.
The type of material also makes a difference because with memory foam you are more “in” the mattress while with more resilient materials like latex you are more “on” the mattress even though both can be equally pressure relieving.
The bedding you use which includes your protector, any mattress pads, and even your sheets can make a significant difference in how a mattress feels and performs and on the ability of any foam layers underneath it to contour to your body. There is more about some of the possible options you can look at when a mattress is too firm in post #2 here.
There is also more about the many different factors that combine together to affect the sleeping temperature of a mattress in post #2 here.
It’s always much better to err on the side of buying a mattress that is too firm than too soft because you can always soften up a mattress that is too firm with a topper but “fixing” a mattress that is too soft involves removing and replacing some of the layers (not adding to them) which is only possible with a component mattress where individual layers can be exchanged.
There is more about the different types of mattress protectors and some of the tradeoffs involved in the different types in post #89 here.
Phoenix