Hi Dian,
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress (which of course there wouldn’t be because your mattress is still relatively new) then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know whether a specific mattress/topper combination is a good “match” for you in terms of PPP is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there is always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, preferences, sensitivities, physiology, and health conditions can all affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) 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 and a link to the online suppliers that have good exchange/return policies as well (just in case the topper you choose doesn’t turn out as well as you hoped for).
There are also some suggestions in post #2 here that may be helpful if your mattress is too firm as well.
A 2" soft topper would certainly be in an “average” range that would work for many people in similar circumstances but of course not everyone is inside the averages so the only way to know whether it will work for you will be based on your own actual sleeping experience.
Phoenix