Normandy and Mt St Michele

You've obviously been to Europe before. Are you sure you're safe to drive after an overnight flight? I wouldn't be. I'm a zombie on my arrival day.

On weekdays I see trains for Caen scheduled to leave CDG at 9:46 AM and 12:38 PM that require a transfer at Le Mans, which would avoid the need to change train station in Paris. Caen is quite close to Bayeux; it's a short train ride away. There are reportedly multiple car-rental companies near the Caen train station.

Given how much driving time is involved in going from Bayeux to the D-Day beaches, I really cannot recommend a half-day tour, though the are offered by Overlord and possibly others. You just won't get much time actually at invasion sites if you don't take a full-day tour.

Everyone says Mont-Saint-Michel is absolutely overrun during day-tripping hours and that it is therefore best to spend the night there, seeing the island late on your arrival day and/or early the next morning. I have never been to MSM so cannot provide personal advice.

I use Skyscanner.com to research intra-European flights.

There is no direct train from CDG to Bayeux, but you can take an RER train from CDG to Magenta and switch to line E and take it to the end of the line getting off at Haussmann Saint-Lazare in the heart of Paris. You can then take a direct train to Bayeux. It might be easier than renting a car and driving 3h to Bayeux after little or no sleep.
Why do you need a tour for Normandy when RSs guidebooks will guide you.? You can download electronic versions from his app. I would rent a car in Bayeux to visit Normandy.
According to RSs 2012 FR guide arrive at MSM late and depart early so spend the night if you can.
Fly from CDG to Vienna (check Skyscanner).

Day 0: I'd probably take a taxi from CDG to Gare St. Lazare and take a train to Bayeux from there. I don't attempt driving when jetlagged.
Day 1 and Day 2: My family took the Two-Day American tour with Dale Booth, pricey but well worth it. Many people recommend Overlord in this forum. We stayed at the Hotel Reine Mathilde and would definitely go back.

We skipped MSM so I can't give you any advice there. We trained to Amsterdam next so I can't advise you on flights.

I probably arrived about 3 PM, nearly 4 by the time I got from the parking lot to the gate.

The streets were quite crowded when I arrived but I followed the Ramparts up to Abbey. The Ramparts weren't deserted but there weren't a lot of people.

I toured the Abbey until just before closing (I think 6 in mid-September) then worked my way down through the center of town. It was still somewhat crowded but not packed.

I had planned on dinner there but the omelettes were way too expensive, so I headed to Cancale for oysters instead.

Thank you to everyone for your valuable insight. I hope we can do this part of our trip. Seems to be pretty complicated to get to Bayeux from CDG- without driving. I agree with not driving after overnight flight.
The planning continues……

I didn't find it too complicated. You leave CDG following the signs marked Taxi, wait your turn for a taxi, ask to be driven to Gare St. Lazare (ignore anyone who offers you a taxi inside the terminal). At the train station find and board your train to Bayeux. The train ride will not be short because Bayeux and Paris are 264 km apart (164 miles).

I don't think you need to book a ticket in advance unless you are traveling at Easter or Christmas.
https://community.ricksteves.com/travel-forum/france/train-to-bayeux-4f97284c-81ce-48fd-9778-4b0ff6aca2a0

You will find it easier to manage a small suitcase than a large one when getting on and off trains. My husband and I take weekend-sized rolling suitcases and visit a launderette or do sink-washing occasionally. This forum has a whole section on traveling light.

This website has maps of train stations: https://www.seat61.com/. For example: https://www.seat61.com/stations/paris-gare-st-lazare.htm

We had a terrific time in Normandy, however you may find there is so much to see and do there that you'll want to schedule it during a trip where you can stay three or more nights. Or as Rick says, remind yourself that you will be back.

Hmm, that didn't carry the tone I was looking for.

Meant to say something like "I didn't find it impossibly complicated" or "it was not a direct shot but I managed it."