You will love the tour! I took it mid-September. I extended my trip by 9 days prior in London and Paris and 5 afterwards in Venice. I’m retired and that timing doesn’t work for many people.
If you can add a few more days to your trip, I’d fly into CDG and spend a couple days in Paris to help get over jetlag. There are more things to do and see to keep you moving. Then take the train to Chartres from Gare Montparnasse; it’s an hour on the regional train. If RS uses the same hotel in Chartres, it’s a 2 block walk from the train station. I went to Chartres a day early but I don’t think that’s necessary since the tour hits the high spots of the town.
A extra couple days in Nice after the tour would be good. Lots to see and do on the French Riviera.