itinerary Provence - 9 days - first weeks of April

If you plan to spend time in Nice save that stop for the end
Drop your car and see Nice for a few nights then fly home from there

We didn’t care much for Aix but have enjoyed Arles, Uzes, St-Remy and L’Isl-sur-la-Sorgue

Since you already have plans to fly into Nice (already booked?), then fly out of Marseille. Also, don't pick up your car until you are leaving the Nice area. You won't need it there (good bus and train connections for the Riviera) and parking is expensive. When you get to Marseille, you won't need it in town. Traffic in Marseille is a big pain , and also pricey. Return the car at the Marseille airport.
You can drive from Nice to the Aix or Arles area. Aix is close to Marseille airport, so drop your car there after staying in Aix, take the shuttle to the train into town from the airport (about 18 minute train ride). Reverse this when you are leaving.
There are many, many villages to visit between your stay in Nice and your stays in Aix and Marseille, so I will leave these suggestions to others. You will have a wealth of options for this leg of your trip. Narrowing down your choices will be hard.
Bon Voyage!

I agree with Judy. I flew into Nice and out of Marseille, spending the last 2 nights in Marseille and thought this worked well.
If you prefer to minimize hotel changes, I would suggest either 3 nights in or near Nice followed by 4 nights in Aix with daytrips or, 4 in/near Nice and 3 in Aix (and then you 2 nights in Marseille). If you don't mind multiple hotel changes, then you might pick a smaller village in Provence for 2 of the nights. With a car, you have many options for great markets and well-preserved villages in Provence. In my opinion, the Nice area is best for major outstanding art collections in small museums such as the Chagall and Matisse museums in Nice, the Picasso museum in Antibes. With only 9 days, you will have to make hard choices.

P.S. I see you have nine days. I hope that doesn't include your arrival and departure days, which would mean you have only seven full days on the ground. Are you counting nights? That makes it easier to plan, as the time it may take to go from one place to another could be anywhere from 3-4 hours to 7-8 hours, limiting your actual "sightseeing" time. (Moving time includes checking out of lodging, getting out of the city, actual travel time, finding your way to lodgings in the next town or city and checking in.) Your trip could be something like this, with the 9 full day options:
Nights 1-3 Nice (2 full days) (Arrival day is night 1)
Nights 4-5 Provencal village, after drive there (1 full day and partial if drive from Nice is not a long one-depends on village location)
Nights 6-7 Aix-en Provence (1 full day, plus a long partial day if you are arriving from a nearby village)
Nights 8-9 Marseille (1 full day plus a long partial day, as Aix is pretty close to Marseille) (Departure is day 10)

If your nine days count doesn't factor in travel time on "moving" days, then you have 2 days Nice, 2 days Aix, 2 days Marseille. Subtract the days where you move (3) and your 9 days are gone.

Of course, part of seeing Provence is in the drive, if planned right, as you can see some lovely villages and scenery along the drive.
Take a look at a map of the area before making your choices. You may decide to drop Aix or Marseille to fit in some of the more "rural" locations. There are markets in just about every decent-sized town. Days of the week vary. Festivals usually happen on the weekends and vary in size. We have been to some very localized ones (Taulignan) and a couple of larger ones (Nyons). We have just happened on these and never planned on attending a festival. You will need to do some research on this and on market days.

Again, best of luck on your trip. Bon Voyage!

Easter is April 5 and that may affect your plans, as school holidays may increase the crowds in tourist locations. Many French families will be heading for the warmer climes of southern France.