“what should have been done instead”
Use the Navies as they had done successfully several other times. Land troops, etc. north of the Rhine between The Hague and Amsterdam. The Germans would not have expected another landing. You cannot tell from Google maps, but the coastline does not look any worse that Normandy.
Landings at Normandy were in fairly close proximity to Britain. Where would the Allies have found either the troops or the transports for such northern landings?