We quite regularly do the run from Calais to the Guipry area, since we are twinned with a town near there, and have found that the easiest way is the A16, A29, A13, A84 to Rennes, and then follow the Nantes signs.
Love driving in France and keep meaning to get a Tag. Will need when we take this over next year!
Friends were cruising in that area in July and had a wonderful time but did get caught out with a failed lock or bridge in Redon which caused them to take a slght diversion.
If you do go to Redon we can recommend the Patates et Campagnie restaurant on the Quai Jean Bart, which is where the public moorings are and the O'Shannon pub by the lock where the canal crosses the river, a short walk away.