How does this itinerary for France and Italy look?
 

Paris is my favorite city in Europe so far and I've spent a week there before so I've already seen most of the major sights(Eiffel tower, Sacre Coeur, Louvre, Les Invalides, Saint Chappelle, Versailles palace, Rodin museum, Notre Dame, Arc de Triomphe, Disneyland, and Trocodero place). I also wanted to spend a few days seeing Venice for the first time, since I've only been to Florence, Rome, and Naples in Italy.

Here's my basic itinerary this time around:
Day 1: Flight arrives in Paris early in day, Montparnasse tower, Pantheon, Notre Dame tower.
Day 2: Catacombs, Museum d'Orsay, Luxembourg gardens, and evening Seine River cruise.
Day 3: Split the day between the 2 Disney parks.
Day 4: Take bus to Mont St Michele and possibly get hotel near there or in Bayeux.
Day 5: Normandy beaches WW2 tour, and American cemetery.
Day 6: Bayeaux Tapestry and more WW2 stuff.
Day 7: Early morning bus back to Paris(or do this day's plans in Paris on day 4 and go directly from Bayeux to CDG airport on day 8), stroll Champs-Elysee and Rue Cler, Picasso Museum, and maybe Orangerie Museum or Pere Lachaise cemetery if there's any time left for it.

Day 8: Late morning or early afternoon flight to Venice, Doge's palace, Piazza San Marco, and Saint Mark's Basilica.
Day 9: Maybe half a day in Padua or Verona, rest of the day exploring Venice.
Day 10: Basilica di Santa Maria, Accademia, and boat tour of Venice. Early flight the next day back home.

I'll be going in late May through early June. At first I was going to give 6 days to Paris and 4 to Venice, then I found out that a place I've always wanted to visit, Mont St Michele, is a doable daytrip from Paris. As I researched that I saw multiple suggestions to give more time at MSM and to leave plenty of time to see the Normandy beaches and Bayeaux. I'm a WW2 buff so I figured spending more time in Normandy and Bayeaux could be great. I wasn't previously aware of the Bayeux Tapestry, so I look forward to seeing that now as well.

A few questions I had:
1. As I've said, I am a WW2 buff, but my main reason for visiting the Normandy area is still to see Mont St Michel. I'd love to be able to see it at night and maybe even see the tide come in. I'd prefer not to be jumping around hotels too much though. Would it be practical for me to use a hotel just outside MSM as a base for Normandy as well? Bayeux seems like it would make the best base and I'm thinking about trying to go directly from there to CDG airport instead of stopping back in Paris.

2. I've read that buses, trains, and even planes are options for getting to and from Paris to Normandy. Would bus be the most cost-effective and convenient option?

3. I've seen a no shorts requirement enforced when I visited the Florence cathedral, but has anyone seen that enforced for the cathedrals around Paris, Normandy, or Venice? I've worn long pants to cathedrals in Paris on my last trip just to be safe, but it would be nice to not have to worry about wearing shorts on a hot day.

4. Any recommendations of good tours for the places I'm visiting?

Any advice appreciated!

I'm pretty sure that the Duomo in Firenze requires no shorts or skirts above the knee, which would mean that you could wear below-the-knee shorts or capri-length slacks. I would assume that St. Mark's Basilica has the same rule, but I don't remember.

I'm not sure why you want to take all those buses, when the trains are more convenient, if a bit more expensive.

Yes, Bayeux makes a great base for exploring the WWII sites, and is a lovely little place in its own right. But you can't go from Bayeux to CDG directly. And if you were thinking of trying to get from Bayeux to CDG for a flight on that same day, don't do it.

Your grouping of sites in Paris seems rather random. I suggest you look at a map of the city and plan more sensible groupings.

All depends on your travel dates. For some of them, a train ride from Caen, or Bayeux etc. to Nantes plus a Volotea flight to Venice (fares start at 15 EUR) might be a good solution