I'm traveling from Pisa to Nice in late august.
Any thoughts or advice would be welcomed.
There were no cross-border trains when I went in 2013, but you can now avoid the need to rebook onto TER at Ventimiglia by using the through trains.
The EC (Marseille-)Nice-Genoa-Milan are operated by thello, a joint venture of Trenitalia and Transdev: https://www.thello.com/