Dinan to Sarlat

All the places I've stayed in that area are probably larger than what you're looking for, but I day-tripped to Chauvigny (pop. 7100), about 20 miles east of Poitiers, and thought it was attractive. Poitiers itself is on the longest of the three similarly-timed driving routes suggested by Via Michelin.

You might want to consider the seaside and use La Rochelle for a midpoint stop.

ViaMichelin tells me going through La Rochelle adds little to the driving time; I had assumed it would make the trip considerably longer. I liked La Rochelle a lot. The historic center is very attractive. La Rochelle is both larger (pop. 75,000) and livelier than Chauvigny. La Rochelle has a good artisanal chocolate shop, Cacao et Macarons, located on rue de l'Arsenal.

What about doing the reverse? Dinan, then Loire Valley, then Dordogne, and you drop the car in Bordeaux for a fast train to Paris, or in Brive or Limoges for a slower train, but shorter drive. You'd need to spend the last night in Paris or at the airport but that's advisable even coming from Loire Valley.
This setup would reduce the total driving distance.

Thanks so much for the responses. And thank you , Balso. After doing some map research, I think we will probably go to Loire first and then on to the Dordogne. Any further suggestions you have are always much appreciated!