Hi Amity,
The upper layers of a mattress are usually the weakest link in the mattress and will tend to soften and break down before the deeper support layers or components of the mattress so your experience certainly isn’t unusual so it makes a lot of sense to try replacing the top layer of your mattress before replacing the entire mattress.
The first “rule” of mattress shopping (or topper shopping) is to always remember that you are the only one that can feel what you feel on a mattress (or a mattress/topper combination) and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or topper or combination of materials and components would be the best “match” for you in terms of “comfort” or PPP (Posture and alignment, Pressure relief, and Personal preferences) or how a mattress or mattress/topper combination will “feel” to you based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
While it’s not possible to make specific suggestions because of all the many variables involved that are unique to each person and your own experience is the only way to know for certain whether any mattress/topper combination will be a good “match” for you in terms of PPP … there is more information about choosing a topper in post #2 here and the topper guidelines it links to that can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of as well.
Assuming that the quality/durability of a topper is suitable for your body type … the choice between different types of toppers would be more of a preference and budget choice than a “better/worse” choice.
Latex is certainly a high quality and durable material that would be well worth considering and a microcoil topper is also be a durable component as well if you prefer the more “springy” feel of microcoils. You can read more about microcoils in this article and in post #8 here and post #2 here.
Again … I would always keep in mind that the only reliable way to know for certain whether a topper you purchase will be a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) in combination with your mattress will be based on your own personal sleeping experience so the return/exchange policy with any topper purchase can be one of the most important parts of the “value” of a topper purchase.
Phoenix