I'm French. As far as I know there is no one in the south of France, and only Dr. Amarenco's service of neuro-urology in Tenon, Paris is really up with POIS (and they're not very helpful). See either Dr. Chesnel or Miget. Paris is not in your area but it's not so hard to go there once for a planned appointment. Dr. Chesnel also offered me to see Dr. Sebastien Beley, another urologist in Paris, if I wanted a second medical opinion on my POIS. I didn't make an appointment.
Other doctors, elsewhere in France, may exceptionally know POIS but probably never saw patients and have a limited knowledge of the issues. I saw a urologist in Paris who didn't have any idea POIS was a thing and told me to see a psychologist. In most cases I would say that urologists are a poor choice if you want to see a specialist for POIS, as they're mostly surgeons who spend most of their time on patients with really different problems. Neurologists would be my first choice. Even if they don't know POIS in the first place, I believe that they can be more useful. Also, most of POIS symptoms seem neurological in nature.
Whatever it be, remember that these doctors don't know what POIS is and how to cure it, and they will only say and try things based on the very lean medical literature on POIS, which you can find on this forum. So, you shouldn't expect too much of them.
Another possibility might be to look for doctors who see patients for Chronic Fatigue Syndrome. I didn't try and don't know any name, but it would probably not be a stupid initiative. The symptoms of ME/CFS are very close to POIS though less linked to a single trigger, and it's much more studied and known more widely. You may ask your GP his opinion.