Sketching out my plan B

I think that Arles could make a nice base for the areas that you want to visit. It's not clear to me how many total days you're have for Provence - six? If that's the case, you might pick a few of the cities that you've mentioned for day trips from one spot. We stayed near Maussane les Alpilles (not recommending it but as an example) and took a day trip to Marseilles to see the city and the museum there. I enjoyed spending the day in Marseilles but I wouldn't necessarily want to use it as a base. Personally, I prefer Arles or Avignon to Aix.

Cassis is a nice coastal city to visit.

Arles is about 3.5 hours from Toulouse.

We based in St Rémy-de-Provence in Sept 2019 and absolutely loved it. We visited lots of towns/cities within a 1 hr drive and liked St Rémy best. We did a day trip to Carcassone and it was fantastic. You will love it. It’s the oldest, intact castle in Europe.

Provence is full of Roman ruins, so you don’t need to include Orange for that. Vaison la Romaine and Nimes immediately jump to mind; and they have other attractions, as well. I would plan on two bases, but it depends on how much time you have and what you pinpoint as your priority sights. My rule of thumb is that a day trip should require no more than 1.5 hours driving each way. And that’s a stretch.

There are hundreds of b&b’s, small inns, and gites available in the region, at all kinds of prices. There are also lots of sources for finding them. When you have a definite itinerary, come back; and lots of posters will share recommendations.

It’s a 2h 45m drive from Carcassonne to Arles where I would sleep. Driving times from Arles to:
Camargue (30-minutes)
Les Baux (45-minutes)
Saint-Rémy (45-minutes)
Uzès (1h)
Pont du Gard (1h) > Nimes > (45-minutes) > Arles (45-minutes)
Châteauneuf-du-Pape (1h 15m)
Orange (1h 15m)
L’Isle-sur-la-Sorgue (1h 15m)
Ménerbes (1h 15m)
Gigondas (1h 30m)
Roussillon (1h 30m)
Lacoste (1h 30m)
Vaison-la-Romaine (1h 45m)
Crestet (1h 45m)
Cassis (1h 45m) > Marseille (45-minutes) > Arles (1h 45m)
Sault (2h)

Thanks @MaryPat, you've done my work! That is typically one of my first steps in planning an itinerary!

@Rosalyn, in terms of day trips we tend to look at whether places are at the end of a "spoke" or if there is a natural route. We don't mind shorter stays, though we like to be at a place for at least 2 days, we hate "backtracking". I'll do an hour one way to somewhere if its something I really want to see and it doesn't land within the planned route. I guess we've done so many road trips in the U.S. that we are just used to changing accommodations. It does look like Provence lends itself to two bases, though most of what we would want to do is more around Avignon than Arles. On the other hand, it sounds like Arles is a better value for inns and for us, maybe a better fit in terms of the city itself. I need to do some study of the smaller towns to see if they might be interesting places to stay.

@Bobby, for me, I'm interested in taking in the places VanGogh walked, lived and painted. I've been lucky to see many beautiful VanGogh paintings in both the U.S. and Europe. And we certainly don't love paying for gas, but we really have very few days on this trip where we drive more than a couple hours. Plus, for some reason (haha) we've saved A LOT of money on gas in the last couple years. It turns out when you can't really go anywhere because of a pandemic, you actually have to make an effort to drive the cars a couple of miles every week.

If we have to use this "Plan B" we are basically slipping Provence inside our trip where we are supposed to be in Spain, if that makes sense. Because I have train tickets and a rental car arranged for my "Plan A" trip, I can't change where we end which is Toulouse.

We used St Remy as a base and we found it worked quite well for our purposes. We used our car for day trips (Arles, Les Baux, Avignon, Châteauneuf du Pape, etc) but, at the end of the day, came back to St Remy, parked at our hotel, and walked to dinner. It’s a very walkable town with lots of restaurants. We never worried about driving after dark, finding a place to park, or driving after that last glass of wine!

@Susan and Mary, it does sound like I should take a look of St Remy for at least some of my nights if not all. I'm assuming there are enough restaurants for six nights?

@Sharon, Cassis does look nice and there is their wine. . .

I've just found the national park by Marseilles. I have always been a flamingo lover. I have never seen them outside a zoo!

That'll put you near Aigues Mortes, I think. Touristy but fun. Can't compare it to Carcassonne because I've never been to the latter.

I've read that you really need mosquito repellant in that area. I walked around a lot in town, but not after dark. I didn't have any insect issues, but I'm one of the lucky people not particularly attractive to mosquitoes.

Ann, I hadn't encountered Aigues Mortes, looks interesting. Thanks for the tips on the mosquitos. Although our state bird is said to be the mosquito, I still react badly to bites. We are also going to bring some tape and mosquito netting for windows in some inns without screens.

I'm getting to the point in which I desperately want to do both the Plan A and the Plan B trips!