We did this journey in June. My TomTom took me on the following well-used lorry route - N176 past Dinan to Jugon-les-Lacs, join D792 and follow this to N164, then D700, D768 past Pontivy to join N24 to L'Orient to take N165 to Concarneau. Good fast route in my experience. We spent 2 weeks in Concarneau on the Les Sables Blancs site and loved it - having really good weather probably helped! We came back via Josselin and Rennes and it was very slow going through Rennes.
John M