Thank you all for the suggestions! Husband mentioned Lascaux because he remembers hearing about it as a child but I don't think he realizes the potential for claustrophobia in the "subway car" lol. I'll research the other options and do some routing. We'll be based in Sarlat for four full days (5 nights), and want to get the most out of our limited time.

La Roque St. Christophe and L'Abri du Cap Blanc are great suggestions for claustrophics! Neither has chromatic paintings, but each is well worth seeing IMO.