Suggestions for planning overall amount of time for trip

So, are you staying 1 night or 2 in Mont St. Michel and Dinan? Will you see Saint Malo? Its close to Dinan.

I think 4-5 days in Paris and 3 days in Loire, is a nice amount of time to get a solid introduction to those areas. Loire is easier/more efficient with a car. (as is Dinan and Mont St. Michel)

Thank you. How much time would you add on for Provence to see the major towns/sites?

I would stay at least two nights somewhere in Brittany; whether Dinan, St Malo or MSM. This past spring we took the train (Montparnasse station) from Paris to Rennes. We picked up a rental car and then drove to MSM. It's much more convenient to have a car. It's very easy to drive; the roads are nicely paved. Drive is scenic countryside.

It's best to plan your travel by how many nights not how many days.
Enjoy!

C.M.,
Provence, not including the Riviera, is about 30,000 square kilometers (about 11,500 square miles). There is such a variety of things to see and places to go that it is hard to answer your question. I would lay out a regular map of the Provence area of France to get an idea of the natural environments involved (Mediterranean coast, mountains (Mt.Ventoux), fields and hills. There are castles and Roman ruins and beaux Villages and wineries and lavender fields, and churches and farms, etc......what do you like and enjoy? "Major sites" don't apply here as they do in, say, Paris or even Brittany or the Loire Valley. They are more spread out, and less condensed.
To enjoy its variety, I would give Provence about 2 weeks, in which you could fit 4 days on the Riviera in some place like Nice. Two weeks isn't much, but I am aware that many of us on the forum feel that there is never enough time, so I am trying to fit my advice to your goals.
Probably, with not a lot of time, you could base in Arles, St. Remy de Provence, or Aix-en-Provence, and do day trips (preferably by car) to the sights like Les Beaux, Nimes, and whatever towns you aren't based in. And do count nights, not days. When you say one day somewhere, that means a day where you wake up and go to bed in the same town. (2 nights = I full day). Travel days will take away at least half of your day. Be sure to factor in your transfer time, including time to check out of lodgings, get to train station (e.g.), travel itself between cities, then get to next lodgings and check in.
What is the total time you have? I wouldn't count arrival day in your plans, or the day of departure. The night before your flight out will be best spent near to the airport, so I wouldn't schedule too much on the day before you leave.
France is wonderful! Whatever you decide, you will have fun. Just find time to relax and enjoy it.

My question is how many days overall would you suggest for us to plan for to be able to go to all these areas without doing so at a break-neck pace

As per a post by the webmaster earlier in the year people have been asked not to critique others plans, but since you specifically asked...

We spent 9 days in Normandy/Brittany and felt we didn't have time to see a lot of what it has to offer. If this was my trip I'd drop the Loire and spend those 3 days in Brittany. Cap Fréhel, Fort-la-Latte, Dinard, St Malo, Rennes, the Emerald Coast a stop in Cancale for oysters on the way to MSM, this just begins to scratch the surface. I suggest getting the Michelin Green Guide to Brittany which is a comprehensive guide to the region.

Or drop Dinan/MSM and add the time to the Loire.

When we went to Provence in 2019, we mapped out our priorities and decided on 6 nights/5 full days based in St Rémy de Provence. That was a good amount of time for us, although one more night would have also been good. If you would like, I can look up what we managed to see each day. We had a rental car.

Edited to add that we then spent 1 night in Moustiers Ste Marie and 3 nights in Nice.