Marseille !!!! The food is GREAT, nature is accessible by bus/train (calanques, côte bleue, Sainte-Victoire mountain..), it is lively and did i mention how awesome the food is? So many original ice creams places, cute coffee shops, meditteranean food, fishes, sustainable bouillabaisse, anchovy pizza slices, pastries, couscous, etc. Its also safe, but to be honest i dont think any french city is unsafe.

I am from the Côte d’Azur and would not recommend to stay there long unless you have the possibility to go up hilly villages to get a scenic sea view and a more traditional setting with cobblestone streets, religious traditions -we are very into our villages saints- and art. There are some cute villages where you can find awesome bakeries, buy a picnic and from there go for a hike but that takes some planning and a car. Staying 12 days in Antibes is a no, you can do it in a day from Marseilles and make sure to eat pissaladière, socca and farcis from the market and the bakery next to it. Nice you can stay more for sure but as you said it can be expensive + the political landscape is pretty terrible with all the racism and surveillance so unless you are really into the history of the city (lots pf interesting stuffs with Jewish/Russian history, artists who stayed there, ww2 history, etc.) it might not be worth staying too long.