9 nights in the South of France

It really all comes down to personal preference, but if it were me, I'd absolutely prioritize the smaller towns and villages. To me you get the more quintessential Provence experience, you're more likely to have personable interactions with the locals, and find little gems that are off the beaten path. When my husband and I go to Provence this June, I had originally thought to stay in Avignon, which I have heard lovely things about, but then I stumbled upon a gorgeous little chateau B&B in a smaller village just outside the city! We will be staying there, amongst lavender and cypress trees, with a laundry machine and a home cooked breakfast included! For the Riviera, since you have a full 6 days, it might be nice to break it up into two different cities for ease of exploring opposite ends of the coast. We will be in Cannes for our Riviera stay, but we only have 4 nights. I wanted to prioritize the beaches, but day trips to Monaco, Eze, or Menton are much further away. Hope that helps!

Marseille is a destination in of itself but I would not attempt to day trip out to Provence. You can then transition to St. Remy or somewhere in the Luberon (where you will find the best change at lavender, at the lower elevations, depending on how warm this spring is). June 23rd or so may be a bit early for full blooming, depending on the spring. Cannes can work for a base -- are you retaining a car? It does tend to be at one of end of the region most people are visiting. Check the location of the train station(s) as it is often more convenient/quicker to take the train to certain points like Monaco or Nice. The main Cannes station is right dead center in the main town.

Manosque is a great place from which to see lavender fields, and it's the home of L'Occitane en Provence. If you're interested in how their personal products are manufactured, a factory tour is available at no cost, and as an engineer I found it fascinating. My wife also loved the experience. Easy to get to by train from Marseille; it's about a 90 minute trip. Getting to Nice from there by train is another matter, but with a car you could travel over to, say, Valbonne for a Provençal town experience and then continue down to Nice.

I agree it depends on your preference. We visited Rousillon in February, and it was lovely. I hear St. Remy is nice but we didn't make it there. Lourmarin is very nice and has a Michelin star restaurant at the resort. Cannes is nice and can be a place to rest. When you're staying there, depending what you want to do, it may or may not be central. Traffic will be horrible, BTW. The beaches aren't particularly nice on the Med. They're not the nice sandy beaches you might be craving. You'll need beach chairs to be comfortable. Nice is very central and you can take the bus and tram everywhere. Recommend the bus for Monaco and you'll get views that you get on the train.