The Atlantic Wall was very well defended and stopped the Allies for several years. Prior to D-Day, the Allies had taken North Africa and most of Italy.
This provided plenty of staging area to invade Southern France and move northward to Paris, Northern France, Low Countries, and into Germany. If Western France could be encircled and destroyed, the Allies would have another landing site closer to the UK.
Was this option considered? From my point of view, timing this with Operation Bagration could have worked even better than D-Day.
The event you are thinking of, Operation Dragoon, did occur beginning on August 15 1944.
The reasons a cross channel invasion was held first were many though. The biggest was that it offered the most direct route to Germany and was the most straight forward to keep supplied. Making Southern France the main effort would have strained the already stressed supply chain in the Med working to keep the drive through Italy going.
The British also weren't crazy about it from the start as they felt it unnecessary drain on Italy and Overlord. But got on board when the plan was to have both landings in France occur at the same time. However the lack of sufficient amphibious vessels meant Dragoon had to be pushed back.