Need help with my France Itinerary...Is this realisti?...Open for suggestions

Doable? Probably. Advisable? Not IMO. I think you are spending way too much time in transit and not enough actually seeing the things on your list - 3 countries in 12 days is quite a lot - bear in mind that you will lose anywhere from 1/2 to 3/4 of a day when you change locations.

What suggestions would you have. We fly into Nice and out of Brussels. I'd love to hear your suggestions of top priorities.

Thanks for your help. I am open to suggestions!

You have 8 locations for 13 nights. Less transit time, you'll have about one full day on average per stop. That's an exhausting pace. Consider limiting your trip to 3 or 4 locations (places you sleep each night).

Perhaps fly into Nice and out of Brussels. Then you could do something like this:

Nice - 3 nights
Paris - 5 nights
Normandy - 3 nights
Brussels - 2 nights

ETA: The French alps are an outlier so I scratched it from my sample itinerary above. Chamonix is great, but there is a gorgeous national park adjacent to Nice. If you'd like to fit some natural beauty into your trip, that would be a good way to do it without spreading your itinerary too thin.

I would drop Geneva/Annecy/Chamonix.

Arrive Nice (3N)
Train to Lyon (1N)
Train via Paris to Rennes.
Pick up hire car. Drive to MSM (1N)
Drive to Bayeux (2N)
Return hire car. Train to Paris (4N)
Train to Brussels (2N)

Why do you think Rennes would be a better train station to go to instead of Caen? I do like these suggestions!

This is all getting there and very little being there. I would skip the Switzerland section and Lyon and spend a bit more time in Normandy and in Paris and while I don't much care for Brussels, what is the point of going there is not to see Brussels? It takes two nights to get a day in a place. So much time and expense on logistics of travel here with so little time to see and do anything.

Since I love the alps I would suggest you drop the d day sites and Mont St Michel (Unless the beach landing sites are of special importance to you). Plus your post doesn't mention what time of the year you are travelling, a lot of choices would depend on that for me. (Alps in the rain is not much fun & except for the Aiguille du Midi most of the lifts are closed off season )
I would do it like this (In fact I did it like this in 2015)
Nice - 3 nights
Annecy - 1 night
Chamonix - 2 nights
Paris - 4 nights ( 3 in Paris and 1 in Chartres was what we did)
Then on to Brussels since you have already booked the flights I guess. If you haven't then you can fly into Nice and out of Paris and do the Normandy sites on the remaining 2 days.
Have a great trip

Hi Margie, I agree that this is too much for the time you have. You'll spend all your time on the train or plane and not really get to see much at all. Arriving in a place in the afternoon and leaving the next morning... not ideal. You need to prioritize what you want to see and drop either Chamonix/Annecy or Normandy.

The first time I went to Europe my friend insisted that this was a "once in a lifetime" experience and we had to see everything (to check it off the list). Well, that made me even more determined to go back and I've been back to Europe ten times since that trip.

While I would bag Switzerland and keep Normandy, Yogi has the right idea -- one or the other. And a bit more time in Paris.

Margie: Rennes has a better rail connection from Paris. It's also a little closer to MSM (though not by much). Pro tip: Catch an early train and stop in Fougeres on the way to MSM for some crepes and cidre near the ruins of the old chateau.