Don’t know that area really well for cycling (apart from once doing an ‘XC’ race around Mt Aigoual that nearly broke me), but you’re looking at two very different geographies : from Millau to Meyrueis/Florac it’s Les Causses, pretty much plateaus and deep valleys so either flat-ish or very steep, then east of there to Anduze it’s the Cevennes, loads of small valleys and up and down all over the place.
If you’re looking to go offraod through the Cevennes then I think progress could be quite slow, but really fun. I’ve driven along the Corniche de Cevennes road to and from Florac, and that looks like an amazing road to ride, but seemed quite long and steep.
Oh and check the weather at the top of Mt Aigoual before planning any camping up there – it can snow in June. In fact, there might be restrictions on camping sauvage and bivouacing as it’s a National Park (usually bivoucing is ok, but check here https://www.lecampingsauvage.fr/legislation-et-reglementation/camping-sauvage-bivouac )