Try this route which worked well for me recently
left UK via vfr route SAM-MP thence to Auxerre (Chartres/Pithevers) to customs in and fuel, customs by notice, two hour IIRC?
Then down the Rhone valley stayng out of Lyons way turn left at Montelemar overflying Provence
Fab trip we went on to Corsica so I cannt help with the that part of the question . We coasted out over St Tropez vor.
Enjoy!