Looking for a stopover location in southwest France

Driving through Tours would only add one hour of driving time, per Google Maps. From Tours, Amboise and Villandry are only about 10 - 12 miles away (in opposite directions); Vouvray is between Amboise and Tours. So, that would not cost you much of anything in terms of your hope to accomplish the drive in two days. Since it would still be a 5-1/2 hour drive from Tours to Bayonne, you would probably want to stop at Villandry or Amboise on your first day, from Mt Saint Michel to Tours. Tours has a lovely midieval town center, as well. The drive from Mt. St. Michel to Tours is only 3 hour and 3 minutes, per Google Maps. So, you could get a nice bit of the Loire Valley.

If you were to leave Mt. St. Michel at about 9 am, you could have lunch in Amboise at 12:30, see Chateau Clos Luce (Leonardo's residence) from 1:45 to 3, stop in Vouvray at 4 for wine tasting perhaps, and get to your hotel by 6. Alternatively, simply have lunch and a visit to the Chateau at Villandry in the early afternoon and an aperitif in the midieval quarter of Tours after you check into a hotel in the mid to late afternoon.

Your 7-1/2 hour route takes you through Nantes & Bordeaux, roughly 1/4 of the way and 3/4 of the way respectively. Rochefort is not far off the main route, and is about 1/2 way.

We visited Bordeaux, the Dordogne, Nantes and the Loire over the course of 3 weeks last spring. Personally, I would take the stop in Tours and one of the afore-mentioned side trips to meet your two-day travel plan. But this is your trip and there is the other, direct route.

I agree that an excursion into the Dordogne would add a couple of days, at a bare minimum.

My first thought when looking at a map is La Rochelle. It's a seaside resort so pretty touristy but it's very nice and for one night it would be lovely, and not far off the main drive.

I do what Nancy does-look at a map and choose a place. She is familiar with La Rochelle, but my choice went to Rochefort. Looks to be a lovely smaller town and is probably not too touristy. I am NOT familiar with it. It does sit about halfway between MSM and Bayonne. Obviously, the two towns are very close to each other.

When my husband and I are going to have a similar driving situation as you will, I look for half way then research a bit to see if there is a place that has any interest and then we stop there. We’ve done that a few times and have always been pleased with the discovery.

La Rochelle and its surroundings is obviously the place where you can stop for a night (or more).
A thousand-year-old city, a pleasant seaside town, just opposite Ré island where you can go by car via a bridge.
Just halfway in your MSM→Bayonne journey.

https://www.holidays-la-rochelle.co.uk/

https://www.holidays-iledere.co.uk/

Going through Tours will add extra miles and take at least an hour longer.

Visiting the Dordogne, even just the basics, requires much more than a day or two

We did the Loire Valley, staying at Blois, which was nice.

Also, consider Bordeaux.

For what it's worth, I 100% agree with JoLui's suggestion. It will break up your long drive nicely and it's a lovely spot.

We stopped in Saintes midway between Pays Basque and Brittany. Easy access, nice pedestrian zone, parks, river walk, Roman arch and theater, and cathedral with crypt. Don’t go if you want to be around a lot of tourists.

Brad