Hi MattressNewb,
Many mattresses on Amazon are in lower budget ranges so they only use thinner layers of memory foam (which is often lower quality as well) so you would sink through the memory foam more easily and feel more of the firmness of the polyfoam support core through the memory foam than you would with thicker layers. Some mattresses also use firmer versions of memory foam than others.
I would be very cautious though about about using other people’s experiences or reviews on a mattress (either positive or negative) as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you and in many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people may be completely unsuitable for someone else to sleep on (see post #13 here).
I would also keep in mind that there are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness as well and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
Yes … you can use a topper to add some additional softness and pressure relief on any mattress that is too firm and is still in good condition without any soft spots or sagging.
Phoenix