Hi dvto2,
Adding a topper is an excellent way to add additional pressure relief to the top of a mattress (typically to eliminate pain, discomfort, or restless sleep caused by too much pressure on a body area). By contrast, adding a topper will not add any additional support to your mattress, and depending on the circumstances can cause you to sleep out of alignment / non suitable posture, which may evidence itself as back pain in the morning.
Anyhow, to help guide you for how to evaluate if you’re putting together a wonderful combination for yourself, this wonderful post by Phoenix helps guide you on finding a suitable mattress for yourself (and yes, I know you asked about toppers, but toppers are a means to modify a mattress to make it more suitable for you):
Also, one of many posts about toppers is here. Phoenix outlines what you’re looking for, the fact that different sleeping positions have different requirements, that you’re best off getting the thinnest topper that does the job, and has links to places to get some.
https://forum.mattressunderground.com/t/serta-icomfort-insight-or-sealy-comfort-series-coral-bay