All of the high points of France can't begun to be seen in two weeks. One issue is whether you prefer to use a car for some of the trip or not. (Not saying which is better for YOU, but some areas lend themselves to having a car.) If you can't bear the idea of only one country, it's early for the season, but look for low-budget flights to Corsica, which would be a big change, and still one country.
We have trained Paris-St. Pierre des Corps and taken a car in a clockwise circle through the Loire, to Brittany, Mont St. Michel, Normandy, and back to Paris. (We'd been to Giverny already, but that's along the way.) Another year, we trained to Strasbourg, and took a car down Alsace to Avignon, and trained back to Paris from Avignon. But these are different styles of travel than you mentioned. You wrote (as I understand it) that you want to focus on big cities or places with lots of good dining and low-key nightlife. We went to two 3-star Michelins on the Alsace trip, but both were by car only, and not in "destination" cities. Do you have plenty of money for food and drink? Are you prepared to reserve weeks in advance for meals? Sorry, maybe you don't want 3-stars!
I would comment that Seville (just one example) is not one of the "great cities of the world", but it has plenty of dining and drinking for a multi-day stay. It's a real problem that you didn't get an open-jaw/multicity air ticket, if you're going to take a chance on multiple countries.
Positano is out of the question, for multi-seat travel reasons. I've been there in the last week of May, and pre-Covid, it was still hard to book less than 9 months in advance, if early in the season. Venice is so important and attractive that you could consider a plane trip there, but the Covid regulatory risk is very high.