Is 6 days in Normandy too much?

Normandy is certainly worth a few days, and its easy to spend 2 or even 3 days on the coast, with visits anywhere between St Mère Eglise and Honfleur. If your primary interest is Juno beach and the Canadian museum, plus it's related sites, you might consider staying in Courseulles-sur-Mer. I think 6 days is a bit much even if you are a military history buff.

Giverny is a couple hours drive from the coast, and while it is lovely in September, there is nowhere close by to drop off your rental car before returning to Paris. I'd actually add a day to your Paris stay and visit Giverny as a day trip.

Belgium is a great place to visit, but there are much more interesting places to visit than Brussels. I'd suggest Bruges or Ghent instead. Both easily reached by frequent trains from Brussels

ETA - re your forum name: any relation?

I.mccrae,
Six days in Normandy is not too much. Six days of only touring the DDay sights is too much for me. Normandy is large and includes many other sights to see and places to explore and enjoy. I found Brussels to be the least interesting part of Belgium, so would skip that. So, with your 6 days in Normandy, I would give 2 full days around the DDay sights (3 nights), 1 full day on cheese/cider route. (Where you spend this night depends on what you find to do. You may be able to do this from your first location, making it Nights 1-4 there.) Drive from there east along the Route des Abbayes, driving next to the Seine part of the way to Rouen. (Stop at the Abbey of Jumieges enroute). Spend 2 nights in Rouen so you can have a full day there...wonderful medieval city with the famous cathedral, food, old town, Joan of Arc connections. Drive toward Paris stopping at Giverny (half-day is good here) and end up in Paris, or drive to Lille to drop car and train to Amsterdam
Night 1-3 Bayeux or Arromanches or St. Mere-Eglise
Night 4 Possibly same as 1-3...depends on cider/cheese decisions
Night 5-6 Rouen
Night 7 Wherever it works to drop the car and catch a train to Amsterdam. (Paris, Lille, ?)

I am assuming you would prefer to avoid the autoroutes (freeways, but not free). The D roads are the most scenic, esp. between the DDay area to Rouen. There are numerous suggestions for Normandy other than DDay sights on the forum. Use the above search bar. Filter for "forum" and "one year or less" or "two years or less".
Have a great trip. Come back to the forum with your more detailed questions as you research your trip. There are many helpful contributors here.
Bonne chance!

Great tips thank you. Having never been there it’s hard to know where to start. I will research based on your ideas and post a potential
Itinerary for more advice.

Another vote for all 6 days in Normandy. Get the Michelin Green Guide for Brittany which covers just about every town, has suggested driving routes, etc. We also used Stu Dudley's Normandy & Brittany itinerary; see https://www.fodors.com/community/europe/normandy-and-brittany-stu-dudleys-updated-itinerary-1670294/

For the D-day beaches I'd recommend taking one of the guided tours unless you feel well informed about the history; the guides at companies like Overlord are very good at explaining what you're seeing and what happened.

I typically spend 8–10 days in Normandy every other year. Yes, I am very interested in history, but there is so much to see other than that, and it is a very peaceful, bucolic place. Kinda like Nova Scotia...

Driving from Paris to the coast, I'd make time to visit both Rouen and Honfleur.

Normandy AND Brittany....I feel that this is too much to cover in 6 days, especially if you want to include Giverny. I would leave Brittany for another trip. Brittany is over 10,000 sq. miles; Normandy is over 11,000 sq. miles, way too much area to cover in 6 days. People often tend to meld the two areas into one lump, but they have stark differences, and each is worthy of as many days as you can spare.
I hope you have a terrific trip!

Would you suggest returning the rental car back to Paris to train to Amsterdam or would Lille be a better option?

I am not familiar with Lille re car drop-offs, but will say that I would avoid driving into Paris at all costs.

We returned our car in Lille last year. The car rental return for all of the agencies is across the street from the train station. Very convenient. One caveat: We returned our car on a Sunday. Hours are very limited on Sundays; so don't do it on Sunday. We had to really coordinate our car return and train to a pretty tight time. But all went smoothly. Then we took the train to Belgium.

Perfect thanks!

We visited Mont St Michel as a day trip when we spent 6 nights in Normandy. So you could add that to your plan if interested.

IMHO this would be a good way to use your 6 days for Normandy: get your rental car in Caen. Split your time between Bayeaux (for Juno Beach and Honfleur) and Saint Malo (for Dinan and Mont Saint Michel). Caen to Saint Malo is only 120 miles. We found the areas I suggest, some of which are just across the border in Brittany, more interesting than other parts of Normandy.

Totally off topic: Lucky you to live in Nova Scotia! We loved our October week there several years ago - Lunenberg, Digby, Wolfeville, and Cape Breton. The highlight was crossing the Canso Causeway with waves crashing -TV news said they closed it right after we crossed!