I actively dislike most castle/palace interiors, so no help from me on the Loire Valley. But I can suggest a few places with picturesque architecture that might be a hit with your wife.
Rouen is a good-sized place that unfortunately took a heavy hit during the war, but they've done a fabulous job of restoring/rebuilding the historic area. It's more or less on the way to Bayeux. There were a good many other tourists in town when I was there in June or July 2017, but they were mostly concentrated in a couple of spots. As I wandered the streets full of half-timbered buildings, I was often alone. Rouen is and feels like a city, albeit a very pretty one.
I liked both Deauville and Cabourg, but they might not be right for you wife if she prefers towns with older architecture.
Fougeres and Vitre are smaller than Rouen and really lovely spots roughly south of Mont-Saint-Michel. I was amazed at how few other tourists I encountered there. I thought they were charming. (I suggest Googling for photos.) Also in that area but either on or near the coast, you have the much more touristy options of Saint-Malo, Dinard and Dinan. On top of MSM that might well be way too many touristy places for your wife, but that issue aside, they are architecturally gorgeous, and there's plenty to keep you busy in that area for 4 to 6 days without going beyond the eastern edge of Brittany. I stayed in Saint-Malo to see Dinard and Dinan and the city of Rennes (handsome historic district) to see Fougeres and Vitre.
If you're interested in the D-Day sites, I highly, highly recommend taking a one-day van tour rather than trying some sort of do-it-yourself approach. Most of the tours originate from Bayeux, and you travel in a small group. With a professional driver/guide you can be sure of not wasting time getting lost and being given information to help you understand what you are seeing. I don't often take day-tours, but I was really glad I had made the decision to do so, because I would otherwise have been bored looking at sand and chunks of concrete. If you primarily want to visit WWII-related museums, that's a different matter.