As for your question: "CP setup for COG next time I'm on the boat. If I can find that and there is a choice between using the sensor or the GPS for COG, what would you normally recommend? And if there is a choice, would my enroute fixit have been to change from one to the other?"

You do want to use the heading sensor (fluxgate) for heading, and the GPS for COG.

They are two different things: Heading is the direction your boat is pointing. COG is the direction it's moving. The GPS is the only accurate sensor for that because it shows your true course regardless of leeway and current.