Marseille to Nice is about 200 km the shortest route. The worst I can imagine would be several hours (say 5) on that very day "everyone" arrive for their vacation at the same time. Normally it is some 2h15 - 2h45 on motorways. Look at www.asf.fr you will find current traffic situation and an hourly forecast on any day forward. The fast route is A8 considerably more inland than the route passing Toulon. The coastal motorway route goes through Toulon at low or very low speed, which reduces the average speed considerably.
The bottle necks are driving out of Marseille to the motorway towards Toulon, maybe some of the toll stations and then driving into Toulon (very much dependent on the time of the day). If you come from Istanbul, I am sure you will find the traffic utmost relaxed :-) all the time.
The coastal road from Marseille towards east passing Cassis, La Ciotat, St Cyr, Sanary, Six Fours and La Seyne is an enjoyable ride if you have time: reserve some 3 hours + maybe lunch at some of these towns/villages. We just spent two weeks down there and got halted by traffic on just one morning, as there was a roundabout in Sanary blocked by the people obviously heading for the market. This was second half of June.
As you will continue from Toulon towards Nice, the traffic on the motorway gets often congested some before Cannes and then also continues that way until Nice. Warning: there may be some very sudden full stops during say 10-20 first kms east of Toulon, if the traffic is congested there. Warning especially closer to Nice the lorries and trucks tend to change from right lane to the middle lane without any warning.
We like Toulon and the surrounding area, and recommend staying around for a while. There are also many nice places accessible from Toulon, both east and west. Hyeres is also a nice smaller town with a big leisure port. The beaches between La Londe and Bormes are exceptional, but also very popular - if you go there, do that as early in the morning as possible.
The coastal road from Toulon until Bormes/Le Lavandou is OK but as you reach Gassin/Cogolin (close to St Tropez) the traffic in core season gets often more like parking Also the connecting road from Ste Maxime to Le Muy to reach the A8 motorway again is often congested, and generally not my favorite, even though the curvy road is a great drive. There is no good way to pass the congested area behind St Tropez other than driving along A8. You can always reserve enough time and enjoy the views on the coastal route. I often miss my automatic gearbox driving a manual rental car there :-) All the resorts Ste Maxime, Frejus and St Raphael + the off-agglomeration points such as Agay and Antheor, between Toulon and Cannes, are generally nice places to see.
As you arrive at Cannes/Nice pick either of them as a base, no point in staying in both of them. An alternative would be Antibes between them. In summer time the traffic makes you spend a lot of time in the car and public parking is in overload, as well. This is why I would also drop the car having arrived in the Nice area.