Hi dfab,
I would probably suggest one of the Brooklyn Bedding mattresses (either on their Amazon site or on their Brooklyn Bedding site) where you can split the top layer. This way you are accomplishing the same thing and have a layer exchange available if you choose wrong rather than having to guess at a topper and then buying another one if you choose wrong (most toppers are not refundable or there can be significant costs involved with a return).
If you do decide to go with a topper then post #2 here and the posts it links to has some subjective guidelines that may help you make the best choice. I wouldn’t add a topper until you had actually slept on the mattress for a while so you had a better reference point about the thickness and softness of the topper you needed. If you have a king size mattress then you could choose two twin XL toppers with different firmness levels on each side and put them in the same cover (and Brooklyn Bedding / Dreamfoam has these available as well).
Phoenix