A county list beats a service area map on a phone
Name the counties in plain text near the top, because a map graphic is heavy, unreadable at phone width and answers nothing.
Write the counties out. Fulton, DeKalb, Cobb, Gwinnett, whichever ones you appear in. A visitor in Marietta wants to see the word Cobb, not a shaded polygon they have to pinch to read.
Say something about travel while you are at it. Whether you meet in the office, by video, or both, and roughly where the office sits relative to the Perimeter. People here measure everything in drive time, and a firm that acknowledges that reads as local.
Do not list counties you do not serve in order to look bigger. It produces inquiries you have to turn down, which costs staff time and generates the occasional unhappy review.
Repeat the list in the footer of every page. It is the second place people look after the header.