must-see towns in Brittany

Based on this new post I would suggest 2 bases. For Dinan, Dinard, St. Malo, and St. brieuc, a good base would be in or near St. Malo, Dinan, or Dol de Bretagne. For Avranches, Granville, Fougeres, and Vitre, a good base would be in or near Fougeres or maybe Pontorson (a bit closer to Avranches and Granville).

If you desire to use just one base, I would maybe use Fougeres or somewhere in that area. It's pretty central and all of the other places you name are <2 hr drive from there with Brieuc being the farthest.

You haven't mentioned Mont St Michel but, if you plan to see that, any of the above bases would work.

I think three bases make sense. One in the north, Dinan or St. Malo. One in the west, I didn't find the perfect base but Locronan is a great town that should be visited. It's been used as a backdrop for movies. Maybe Quimper or Concarneau? Then a third in the south. I really liked Vannes, Auray seems to be a smaller version. I visited a series of small towns to see the churches a little west of St. Brieuc. I enjoyed the area but the towns are pretty quiet and isolated. I'm not sure most people would appreciate the area unless they were visiting a Pardon.

Just another comment about the number of bases. If you end up with 7 days, then 3 bases is too much and 1 or 2 would be better. If you do have 10 days, then 3 bases might work for you. It will really depend on how much driving you want to do over the 10 days, how far apart your bases, and how many parts of Brittany you want to see. The driving in Brittany is generally pretty good, not horribly crowded with traffic except in the larger cities, and even then it's not like Paris or anything. And driving from one base to another will give you opportunities to have a quick stop or two between for lunch and sightseeing.