Hi chip,
What I would do would depend on what was available to me to test and how it compared with any other options you may have.
If you can test a mattress or a mattress/topper combination in person and verify with confidence that it fits your needs and preferences in all your sleeping positions (and this may involve spending enough time on the mattress or combination that you could tell with reasonable certainty), then that is the direction I would go because it’s less risky than trying to add a topper that you haven’t tried. If this isn’t possible or there aren’t any specific mattresses (or mattress/topper combinations) that you test in person that appear to work well for you … then I would add a topper based on the guidelines in post #2 here and the posts it links to.
Phoenix