Hi! Welcome to the forum! I haven't walked in September, but understand that it can be quite hot. You might want to consider starting late in the month - both for reasons of crowds and for cooler weather. Of course, none of us knows what the number of pilgrims (or the weather) will be like next year. I have walked in October-November and quite enjoyed it.
When you say you want to start in France, do you mean to start at St Jean Pied de Port (SJPP)? Many people choose to start at Roncesvalles on the Spanish side of the border, or even in Pamplona or elsewhere. It is simply not true that you must or should start the
Camino Frances at SJPP. It is misguided to have the idea that you must start there in order to do a "whole Camino." You will understand that better after a few days of walking.
The full day from SJPP to Roncesvalles is somewhat arduous, especially if you are jet-lagged and just getting your legs going. You have a choice of two routes:
- Napoleon route that goes a little higher over the Pyrenees, past the Orisson albergue (mentioned in the post above). The views are great, IF the weather is good, but the first 8 km might be the hardest section you encounter on the Camino Frances. I find that as long as I walk very slowly, stop every few minutes for breath, and avoid trying to chat to other people, I am fine. But I don't generally have a problem with overheating.
- The Valcarlos route through the town of Valcarlos, where there is an albergue and other lodging. I haven't walked it, but those who have say it is lovely. It gives you two slightly more even days, and more flexibility in last-minute accommodation decisions.
The number of beds is limited at Orisson and you must reserve well in advance because they are usually fully booked in September.
There are various ways you can manage that first stage - for example taxis are available from SJPP to Orisson in case you don't want to do 27 km to Roncesvalles and you can't get accommodation at Orisson. You can walk there one day, get a taxi shuttle back to SJPP, and then get transport back to Orisson the next morning. (This is very easy to arrange, and you will find you have lots of company.) You might also arrange to have your backpack transported that day, even if you never expect to do it again.
There is a lot of information here on the forum about what the route and facilities are like. However, there are so many unknowns about the post-Covid situation that it is very difficult for us to make planning recommendations. Even so, we will try to answer your questions!