Just got back from a journey and we took the Via Fluvia. We were three of us : two parents and a 7 year old with his own bike. There are numerous stretches of this via Fluvia that are too dangerous for a family or even for a slow cyclist with a heavy load : heavy traffic roads with lorries. We took our MTB instead of road bikes and diverted through tracks on the GR65 Compostela hiking trail. Most of the time we could ride without issue and we were carrying a full equipment 😁 we had to push a little between St Sauveur and the Col du Tracol. Doing so adds a significant elevation to the road.
Otherwise, the Route de la Gance as a junction between the Via Rhona and the Via Fluvia is a beauty and the cycle path between the Col du Tracol and Yssingeaux is really fine.
All in all, I would say it was really beautiful but had to be creative to avoid the traffic between Annonay and the Col du Tracol.