Walked from Canterbury to Martigny in 2022 and Martigny to Lucca in 2023. As noted by others, the path across France wanders around, particularly in the Western parts of the country. A host explained to me that the French folks who use these paths for weekend and day hikes do NOT want to walk on tarmac roads, so the GR route in France "will gladly add 10K in the forest to avoid 400 meters of asphalt!" To wit: my route across France, following as closely to Sigeric's route as I could, but using a combination of Paul Chinn's excellent book, TimR's notes from the Confraternity of Pilgrims to Rome and an app called Organic Maps, all combined to eliminate almost 150 KM from the official route as far as Lausanne.
Note: once I arrived in Switzerland, the route became much more clearly marked (and prices of everything were 15-20% higher than in France). And once I crested the top of the Great Saint Bernard Pass (about where I started to consult Gronze), there was a lot less wandering around. Of course, the Val d'Aosta had some changes to the route due to flood damage and a couple of trail washouts, but they were reasonably well marked and I had no trouble walking through to Lucca.
I plan to resume and finish the route to Rome in late April/early May of 2024. Also, as noted above, you will meet few other pilgrims from Canterbury to Great Saint Bernard Pass (I met a total of 15 to that point in my walk), but met dozens more after I left Aosta, Vercelli, Piacenza and Pontremoli.
Sandy Brown's book was a bit out of date on the part from Lausanne to Lucca, and when I noted route changes to the good folks at Cicerone, they said an updated book was in the works. Not the author's fault about some of the route changes, as local society members and the occasional farmer will close or alter a portion of the official route with little notice (although they do occasionally post little "VF" or "Pilgrim" icons with a small arrow on fence posts or utility poles). A compass and map app are helpful in such situations.
Bom Cammino.