Hi vegister,
There is more about the effects of thickness in post #14 here.
The suggestions of an online retailer or manufacturer are based on their knowledge and experience over the years, the information you provide them about your body type, sleeping positions, preferences, and circumstances, and on the “averages” of other customers that are similar to you and the combination of layers and firmness levels that they would generally do best with.
Having said that … not everyone fits into the “averages” of other people and your own personal “real life” experience will always “trump” theory.
If you hardly made any impression in the mattress then this could be an indication that the mattress may be too firm (a mattress where you sink in more deeply into the comfort layers and contours to your body profile more effectively may be more pressure relieving or “supportive”) and may be an indication that an additional layer may work better for you.
I would certainly give any mattress more time than just a few days to decide whether it is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) because it can take some time for you to adjust to the feel of a new mattress or for you to get a better sense of how a mattress will sleep or any “symptoms” you may experience over the longer term in terms of pressure relief or alignment but if a mattress is working well after about a month and you are sleeping well without any symptoms and you are happy with how it “feels” then the odds are good that it would be a suitable choice in the longer term as well.
Phoenix