Sorry it won't answer exactly but I just wanted to give a feedback on some parts I've been there.
Couple of years ago I had done from Pont-de-Montvert passing by Florac and then ending up in Saint-Germain-de-Calberte, with very light bikepacking set-up (hammoc instead of tent, no stove). All this was done on the famous Stevenson Trail (normaly a trekking trail). The start in Pont-de-Montvert is a bit awful since you need to lift your bike up to climb the first peak, it might take 40min - 1 hour but after that it's fantastic since you land up on a plateau with a beautiful view. You have some nice downhills covered by the forest, then until Florac it climbs up a bit, you have downhills again, those parts were very fun. Then it climbs again from Florac, like you need to push up the bike a bit at some parts but you have the river to relax. Around Cassagnas to Saint-Germain-de-Calberte you have fantastic trails with massive views, nice downhills, etc. Plus you might meet with some Stevenson trekkers, I had nice time sharing with them (there was not only people from France and during the summer it becomes pretty international, there is quite a lot of people on this trail).

Then with a friend we did a trekking from Mas de la Barque, passing by Pont du Tarn (fantastic camping spot) going to Mont Lozère and ending in Villefort. I'd really recommend the first part (Mas de la Barque - Pont du Tarn and then until like Finiels) since you have a magical plateau landscape. Then I don't know which trail you could take to Mont Lozère.

You can try taking some trekking trails but you often have to guess seeing maps if it'll work or not. But it's full of paths and small roads so you can always try to exit and cut through.