Just an proposal, for 2018 mountain bike vacation plans:

https://www.gtj.asso.fr/en/routes-and-activities/gtj-mountain-biking-route/

Did that years ago. We started in Saint Hippolyte and from there on mountain bike trail “GTJ” to Le Grand Abergement. Then we biked back on small roads to Saint Hippolyte (on shortest road route possible, was a blast as well. (We had the car in Saint Hippolyte)).
The GTJ for mountain bikes follows more or less the border to Switzerland.
Good physical fitness necessary – but the tour can be easily done with beginners mountain bike skills. At the time we made the tour we had simple XC hardtails as well.
Landscape fantastic. Lodging sometimes very difficult to find. People very friendly and all talk English as well. You see maybe every two days some other people biking the GTJ – means no crowds at all! Saint Hippolyte a good starting point (from there the landscape is great). Public transportation extremely bad – we biked back to the car (that’s a quick ride on roads. Lot’s of downhill!).
We bought via amazon the maps for “GTJ / VTT”. Doubt if this is still available. Guess bikers use today GPS data.
August is a perfect time for the trip.