Hi azngeek,
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress and it’s still in good condition (which would be no problem with your mattress because it’s new) then a good quality topper can be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) is based on your own careful testing or personal experience on the combination. If you can’t test the mattress/topper combination in person then there will always be 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, and preferences can affect which specific topper would be a suitable choice for any specific person on any specific mattress.
There is more information about choosing a topper that also includes a link to the better online sources I’m aware of 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.
Because of the uncertainty involved with purchasing a topper where you can’t test the combination in person … a good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case the topper you choose doesn’t turn out as well as you hoped for.
I would agree that a firmer topper would likely be too firm for your wife and it sounds like you are getting some good advice.
Phoenix