Oh, there is so much to see!
You are probably aware, but Easter in 2027 is March 28. Easter Day may have some impact although most museums will be open. It just may be more crowded.
For small museums....have you done:
Marmottan Museum (LOTS of Monet's work if you enjoy him!)
Musee Andre-Jacquemart
Cluny Museum
The Museum of the Liberation of Paris - It's a city of Paris museum so it's free but closed on Mondays.
https://www.museeliberation-leclerc-moulin.paris.fr/en
Army Museum (suggesting this since you went to Normandy before)
Musee Quai Branley
Museum of Jewish Art and History. I find this museum very hot but you probably will not have that to deal with in March.
Museum of Hunting and Nature
Musee Cognacq-Jay
Musee Carnavalet
Musee Malliol - if they have a good exhibition or if you are a sculpture fan
Museum of Decorative Arts (next door to the Louvre - I found this surprisingly crowded when I visited in April of this year)
Biblioteque National de France - Richelieu. (I had never visited this until last October and wow, it is excellent. Like a mini Louvre but not crowded and small enough to not be overwhelming!)
https://www.bnf.fr/fr/la-bnf-richelieu
Basilica Saint-Denis (this may be affected by Easter services and possibly for a few days before for Good Friday and Easter Saturday)
Do you have any specific interests? WWII? A specific period for art? Cemeteries?
Have you ever done a Paris Walks walk? They are generally theme based and if your husband is a WWII buff you might enjoy the Right Bank walk that is held weekly. It is excellent.
Chantilly makes a good day trip as do some other chateaux - Fountainbleu and Vaux-le-Vicomte.
There are also a number of sites I've not made it to:
American Cemetery of Paris located in the suburb of Suresnes.
https://en.wikipedia.org/wiki/Suresnes_American_Cemetery_and_Memorial
Malmaison - Napoleon bought this for Josephine
A few years ago I went on a hunt for Roman sites in Paris. Of course I knew about the Arenes de Lutece and the baths at the Cluny Museum but I found a number of websites that listed some Roman sites I did not know about. I built myself a Roman Paris walk which was fun. I'm happy to share if that is of interest to you.
Last trip I also did a ChatGPT query to see if there were Egyptian themed sights to see in Paris. Of course the obelisk at Place de la Concorde is what got me started but it turned up a few sites for things that were Egyptian related, most built in an Egypt-themed revival at the end of the 19th century. I am not a big AI user but wanted to see if this would get me anything more than I could google up on my own. I'm mentioning it in case either of you has an interest in something esoteric.
PS - I would plan to be in Ghent for the tour at least the day before. I found there was a lot other see in Ghent that is not covered on the tour so extra time there is good PLUS it gives you some flex in case of travel hiccups.