Proposed France trip next May

If you are arriving from home, I think it's a poor plan. Driving jetlagged and sleep-deprived is the same as driving drunk. Instead you could start with your days in Paris, possibly including a day trip to Giverny by train. Then go on your road trip.

Sleep in Paris, then rent a car.
Or, take the train to Giverny and rent a car and go.
No matter if you can sleep on the plane, it's not fair to others if something goes wrong.

Or take a train all the way to Honfleur. Spend the night and pick up the car in the morning. It's not far to Bayeux. The nice thing about the train is you can nap on the way rather than trying to force yourself to stay awake and concentrate on driving.

I feel the same on the way back in. Why not turn your car in when you are done visiting Chateaux and train back into Paris?

You want the car for both Normandy and Loire, but save yourself some wear and tear by using the train where it makes sense.

I think your plans are mostly fine. If it were me, I would reverse the first half of your trip as there's a great tgv from Cdg to Tours from which you can then rent a car. I've done this twice before and it worked fine. Tours is an easy 25 minute drive to Amboise. Your first day can be Chenenceau and evening walk in Amboise. I would not spend the first night in Paris.

Also, I would consider giving one night in Paris to changing your one night in msm to two nights in Dinan. You would drive to Dinan, see Dinan, do the Rance river walk. Then on the next day tour saint Malo in the morning, drive along the emerald coast to the sculptured rocks, lunch in cancale (oysters) then arrive at msm around 3, when the crowds are less.

Matt has a good idea about taking the train to Tours. There's no train service to Honfleur.

Previous responders have made good points about reversing the order. Take the train to Tours, rent a car, and start out from there. Although I have stayed on MSM overnight, it may not be the best fit for your itinerary. Perhaps depart Loire Valley and swing by MSM to see it during the day (yes, there will be crowds) and then continue to Bayeux. This will give you more time to see the D-Day sites and enjoy the town of Bayeux--to include the Bayeux Tapestry--a must see! You may consider returning the rental car to Caen and then train to Giverny and onwards to Paris. This would mean bypassing Honfleur; but, the world will not come to an end if that occurs.

Honfleur is worth 90 minutes at best.
You will need at least 2 hotel rooms, wherever you decide to go.
If you can find a "gîte" outside any city that would be great.
In Paris, you should book 2 rooms at the Citadines at Richard Lenoir. The Bastille market is very close by and transportation options are very good.
You'll have a kitchen, once-weekly maid service and laundry facilities on-premise.
Booking ahead can give you discounts.

Thanks to all who took the time to read. Taking the advice the revised plan is:
Arrive May 17, train from Charles de Gaulle to Tours rent car, Château of Chenonceau overnight in Amboise.
May 18, Château of Chambord, Château of Cheverny with possible Château Royal d’Amboise, Les Caves DUHARD, Château of Clos Lucé overnight in Amboise. (Other wine tasting?)
May 19, Château of Azay-le-Rideau, Dinah overnight in Mont St-Michel or nearby.
May 20, Mont St-Michel in am, overnight in Bayeux
May 21, D-Day sights, overnight in Bayeux
May 22, visit Giverny, return car at Airport or somewhere else? Paris

How many Chateau's in a day is too much?

Thanks again.

I think you're wise to follow Matt's suggestion, take the fast train to Tours and avoid driving on your arrival day.

May 18 looks too busy, you may want to pare it back. I think two chateaux is enough for a day, but your mileage may vary.

On May 22, see if you can return the car in Vernon, which is the town nearest Giverny with a rail station, then take the train on in to Paris. No need to use an airport just to return a car.