Having already walked the Via Francigena to Rome in 2022 and from Arles to Santiago in 2018. I looked into part of the route you are considering in an effort to complete the crossing from Rome to Santiago in 2023. However after a good look at the route along the Italian and French coast I learned that it very often involves cement and paved roads through some very touristy areas. I don’t mind the road walking, but decided to walk the Way of St Francis instead, also a decent walk though more challenging than the coastal route I was considering. I’ve walked distance in France, Spain and Italy many times and always find France more expensive than the more southerly routes. Sorry, I can’t advise on pilgrim accommodation availability, but I would guess that it’s not what you are use to from your two walks in Spain. Not enough foot traffic to find dedicated pilgrim accommodation. The tourism along much of this stretch is pretty busy, I think more so than what I saw along the Norte, meaning that you will likely have to book well ahead as you go. Hope you figure it out and that you post a bit of what you learn here on the forum for others. Enjoy!