Brittany is pretty easy to navigate. I did use my smartphone with a GPS app but there are only a few main roads that take you where you are going and the signage is decent.
I took time to test my smartphone's capability to navigate without data in Brittany. I found that you often need data to search for an address but once you have your destination figured out, you can turn off data and get there using only the phone's GPS. Conceivably you could use WiFi to search addresses for your planned stops then navigate without needing data or a local SIM card.
I used two different navigation apps - CoPilot and Google Maps. I liked CoPilot much better. It gave me the speed limit, speed limit warnings, speed camera warnings and looked and sounded like a regular GPS. It cost $30 to download western Europe maps (I did this at home before the trip).
I liked Google Maps better for searching, it was much quicker, but I didn't like the navigation as much. The voice was too fast and hard to understand and the map didn't resemble a normal GPS. Also no speed limits or warnings. I ended up searching with Google Maps then entering the address manually in CoPilot to get there.