Most doctors make good money, but most who are good doctors could make much more.

Simple example pitch that many general practitioners might consider using (based on local laws, of course):

“Insurance covers C, Y, and Z. I can offer these additional services for A, B, and C that are not covered by insurance.”

Where I live, you get a lot of this via “concierge doctors”, but that system can go far beyond basic concierge service, and people are willing to pay for top quality care.