There is a bus from Rennes to MSM - easy and inexpensive, and says you the trip all the way to Caen. You to visit Caen AFTER MSM! I highly encourage you to rent a car, though, if possible, as Normandie is full of history - religious, medieval, WWII, and so on. Driving is by far the easiest way around and in the Fall of 2021, we found the roads fairly empty and parking available even in the bigger cities.
Finally, I highly recommend Appletree Gites (
www.appletreehillgites.fr) if you want a place to stay that is central to everything you’d like to see there. Accommodations for couples as well as large groups (and even a “pilgrims’ room since it’s on the route from Caen And served as one hundreds of years ago!!!), the owners are from England and quite lovely.