Answer the address question in the header: city, county, St. Charles or Illinois
The first thing a homeowner wants to know is whether you cross the county line for their street.
Put the service area where the rotating hero image usually goes. Not a vague sentence about serving the greater metro, but the actual list: City of St. Louis, St. Louis County, St. Charles County, and the Illinois side if you serve it.
Naming city and county separately is not pedantry here. They are different governments and locals treat them as different places. A site that writes only "St. Louis" reads like it was built somewhere else, and buyers notice.
If you cross the river, name Belleville, Edwardsville and the counties you cover. If you do not, say so plainly. A clear no saves your office a dozen calls a month and costs you nothing you were going to serve.
Service area pages should follow the same rule and go to municipality level, because the person reading them thinks of themselves as living in Kirkwood or Florissant, not in a metro area.