Small scenic town to stay in on coast between Nice and Marseilles?

I haven't driven that route. Trains are incredibly convenient along the coast. Are you sure you don't want to take advantage of having a car to see some of the inland places that are not so easy to reach by public transportation? If you're unlucky, you could hit some really ugly traffic as you head west from Nice. I was told it was unpredictable.

One possibility might be Hyeres, but the town itself isn't really what I'd call "small". I thought the historic center was quite attractive. It's way past the halfway point, though.

I would recommend Bormes les Mimosas or nearby Le Lavandou. The former is a pretty hill village, the latter is a small coastal town.

VIllefranche- Sur-Mer
Is just what you need / it’s just East of NICE
in fact it’s the next train stop after Nice in you are
coming from Cannes
It’s low key and relaxed , but it’s pretty as a button
and less expensive than it’s neighbors
We visited there in June 2019 and enjoyed it much more
than Antibes
Emilio CUZAN ecqzan@aol.com

Cannes is a good candidate as long as it’s not during
the annual film festival in early May
We stayed at the wonderful Hotel SPLENDID
it’s the best kept secret in Cannes
Enjoy !
Emilio Cuzan ecqzan@aol.com

Marseille to Nice is a 2-2.5 hour drive. Why do you need to stop for a night? And like others, I don't see the need for a car on the Riviera - trains are excellent and parking is difficult.

With a detour (bringing total driving time to near 4 hours) to the coast, Saint Tropez is about half way between them. I haven't been there.

Thinking through this again, if you have two days to drive between Nice and Marseille, I would not follow the coast. Instead, I would pass by the Verdon Gorge, and stay in that vicinity: Moustiers Sainte Marie, Aiguines, Aups... Dramatic landscapes - more so than the coast! And beautiful villages to match (for instance from Nice you can stop by Gourdon, Bargeme on the way to the Gorge)

St. Tropez and Ramatuelle (and the peninsula) in the bridge season is quite nice. The traffic to/from the A8 tollway can be difficult/busy. I'd duck in and follow the coast from Mandelieu/Cannes and rejoin at Hyeres. Besides the other spots mentioned, further down you have Sanary-sur-Mer.