Suggestions for travel to northwest of France.

The Olympics will be on: avoid Paris, and perhaps Marseille too (nautical events there).
You mention seaside: would lakeside be OK? In which case, Annecy would be a lovely base. Low on art, but some history, and plenty of cute villages (car needed): Yvoire, Le Grand-Bornand, Alby-sur-Chéran... The mountainous countryside of Savoy is really beautiful.

To get there, you could fly from Rome to either Geneva or Lyon.
The train connection to the south won't take too long afterwards.

Thank you I will check that out. I was thinking more of the west coast but Annecy sounds lovely!

I don't know enough about any specific areas--except around Nice--to suggest a place that I think would work for the full two weeks, but I spent multiple nights in each of the following cities during a rail trip in 2019. They were all very interesting, and I'm sure there are good side trips form each of them--I'm just not sure how many. Listed from north to south:

Quimper (Brittany)
Vannes (Brittany)
Nantes (formerly Brittany, now Pays de la Loire)
Poitiers (Nouvelle-Aquitaine)
La Rochelle (Nouvelle-Aquitaine)
Limoges (Nouvelle-Aquitaine)
Bayonne (Pyrenees-Atlantique/Basque Country)

You could also explore the Dordogne and Lot, but I wouldn't know what to suggest as a convenient base or two with a car. I needed five different bases because of fairly thin rail and bus networks in that area.  But that's definitely not northwest France; nor is Bayonne. I don't know where to draw the line there, though the Breton places would clearly qualify.

Thank you very much. I so appreciate the time you both took to respond to my question.

The primary challenge is that fast train options from the northwest to Provence are non-existent. It will be a slow journey south, or a TGV from someplace like Rennes to Paris Montparnasse, change stations to Gare de Lyon (Paris), then down to Provence. I have no idea how the Olympics will impact train travel, but otherwise this route is relatively straight forward.

Rennes has an airport (Rome is listed as a destination) and should be a decent regional transportation hub. Driving in the area is enjoyable. The countryside is beautiful, dotted will many small villages. You can explore places like Bayeux, Mont St-Michel, and Saint-Malo (setting for the novel: All the Light we Cannot See)

Alternatively, you could fly into Rennes, pick up your car, spend just a week here and then take a slow drive south and explore the Loire or Dordogne region on your way. Since you are a confident driver, this is probably what I would recommend.

I have no idea how the Olympics will affect places outside of Paris, you'd have to do some research before making a choice. However if that doesn't look like it will be a problem in Brittany I can say that I could easily spend 2 weeks in Rennes. I loved the city itself and there are tons of good places to day trip to from there by train, bus, or car, including the coast and places like Dinan, St Malo, Mont St Michel, Fougeres, etc. Train from there to Marseille would be about 6 hrs with change in Paris or you could fly from Rennes to Marseille in a bit less time (probaby about 4 hrs). Lovely countryside and quaint villages in that part of Brittany.

Regarding northwest France this summer: I'm working on the assumption (it could be totally off also) that going to Normandy in May and June (my allotted time frame) from Paris will be packed or next to impossible especially on week-ends.

To reduce the odds of this happening, I intend to take the TER train Paris to Cherbourg, always wanted to visit that city not only for what happened in 1944 in the battle for Normandy but also because of the famous French film connected with that city.

While in Cherbourg a couple of days trips to the Normandy towns are planned, ie less of a chance that approaching these towns, Carentan, Bayeux, etc from this opposite direction will see TER trains packed or even sold out, which I saw in July 2023.

I would avoid any itinerary routing through Paris in late July, early August. Train connections between Brittany/Normandy and Provence all go through Paris; unless you use the single Le Havre-Marseille train or the handful of Rennes-Lyon trains, I would avoid it.
Bordeaux to Marseille does not go through Paris.
I would also avoid flying from Rome to Paris.

So, if you find flights from Rome to, say, Nantes, Rennes or Bordeaux, and take care to avoid Paris on your way to Provence, then why not try the west coast!

Rennes is an excellent idea if you don't mind being inland. In addition to the places already mentioned, you could visit Vitre and Dinard.

These are wonderful suggestions! Thank you so much everyone. I'm scrutinizing a map, airlines and flights to see what might be feasible. Your input and ideas are greatly appreciated!