Where to Stay Between Mont St. Michel and Mantes La Jolie

I wouldn't call Caen "touristy" at all. It has a few sights of interest to tourists, including the large Caen Memorial Museum, but it's mostly a modern, industrial city. It wouldn't be my choice for breaking up the drive. My experience in that area involved train and bus transportation, so I'm not a good source of info on overnight stops convenient to the highway. I can say that I enjoyed walking around Cabourg and Deauville. However, those coastal towns are a bit off the highway, and I have no idea what sort of parking challenges they might present.

I hope some folks more familiar with Normandy than I am can suggest an attractive small town or two not far off your route.

Ah option could be to return the car in a different location, normally that's not an issue. Caen or Rennes work, St Malo as well but options will be more limited.
You then take the train from those cities back to Paris.
If you choose Rennes, allow plenty of time to return the car at the station: the car rental area is at the top of a 10-story car park, and the route to the car park is quite circuitous coming from the north (as you will).

On the way back you can visit Beuvron-en-Auge east of Caen and about 15 minutes driving from the main road. It’s a very charming place, so very popular and expect it will be busy there during the weekend. Know you have to exit the A13 freeway just before the toll plaza for driving to Beuvron-en-Auge

Hon fleury is a charming , historic town.

I agree on the visit to Beuvron-En-Auge……such a charming place and Honfleur should not be missed!

I'd agree with acraven that Caen isn't a great stop for sights or ambiance. But it is convenient for trains and rental cars. I picked up a car in Rennes once on my way out of Paris heading for MSM, and it was a wonderful city. I don't remember any hassle picking up my Hertz car, and in fact, it was on the ground level right next to the train station, and the other rental agencies were there too. While I can't be certain, my guess is it is a large enough city to have virus testing. Plus, there is a direct, high-speed train leaving Rennes for Paris (Gare Montparnasse). Other than that, I highly recommend Bayeux over Caen, and you can either take short drive to drop off your car in Caen (right near the train station). Or drop it off in Bayeux itself (not near the train station) and take the train from there.

Rennes does have a nice historic center. It's a bit of a walk (though a pleasant one) from the train station.

Compared to the other places you've mentioned, Caen is probably the least "touristy" of all of them.
The central town is nice for strolling. There are remnants of an 11th century Norman castle in central Caen (it's quite large -- built by order of William the Conquerer), and some impressive governmental and religious buildings. The park and gardens behind l'Abbaye aux Dames provide a nice place to walk among the greenery.