Simon, last year my SLD (Speed Limiting Device) and I toured down through France and Switzerland into Italy to the Lakes for 8 days. We did it in early September. It was a wonderful time to go. The weather was just right, the schools were back so no rug rats getting in the way and few cyclists or camper vans. Once we got to Colmar for our first night, the hood stayed down the entire time, even at the top of the Gavia pass at -1C in a metre or so of snow. We did ten Alpine passes in all, mostly lesser known/used ones.
On our last day we stopped in Epernay for some lunch (with champers too, of course!) and then put the hood up for the long autoroute flog along the A26 to Calais. I hasten to add that we were driving our Jaguar XK convertible, not the Sprint. Much as I would have relished chucking the Sprint around some of those passes, the SLD was not going to spend any time fitted into the Sprint passenger seat, oh, no! I have to say the XK was a lovely grand tourer to do the trip in and, dear reader the SLD and I shared the driving.
The planning was a fun part of the expedition. There are many online recommendations about the best Continental driving roads, which will be helpful. A good paper map is, in my view, essential, as well as a Waze or Via Michelin app on the cell ‘phone.
We chose to go via the Eurotunnel. It has since become ridiculously expensive, especially if, as we did, we had a FlexiPlus ticket for the return journey, which meant we could rock up at Calais at anytime on our chosen day, stuff ourselves full of delicious French patisserie and other good food, also stuff a doggy bag with same, then get priority boarding and disembarkation. If I was doing it again, I would look at some of the ferry options from/to the less popular ports. I hear they are better value nowadays.
The other bit of advice I would proffer is to get to/from your chosen good driving roads as rapidly as you can, using the French autoroutes. It will give you that much more time to explore the regions you wish to, at your leisure. We used booking.com for all our hotel stops and excellent service we got too. It would get to around 1600-1700 hrs and we would halt, look at the map and decide where we were going to stop for the night and then consult Tripadvisor/Booking.com to ring ahead and book in. It worked very well.
Tim