I am not sure if you are starting your Camino mid-September or arriving in Santiago. I finished my 5th
Camino Frances on Oct. 23rd this year. In any case here would be my advice:
1)The fall is actually a very busy time on the Camino and the numbers are still high in September.
2) If you want to avoid crowds do not stay in the towns that the books recommend and do not arrive at one of them between 8:00-9:00 a.m. I stay off-stage but this year hit Portomarin at 8:30ish and walked with between 100 and 200 people AT ALL TIMES that day. I would walk faster only to find more people. The next days we planned our schedule to not only stay off stage, but not arrive in those towns at those hours and it was considerably quieter. Keep in mind that the sun rises very late in the fall when you plan your days.
3) Watch out for Spanish holidays when many Spanish walk. In the fall, I believe they are Oct.12 and Nov. 1 but there may be local holidays as well. I have found accommodations difficult on the Camino around the holidays.
4) Santiago is a tourist destination also, so not arriving on a weekend would probably be best. Also, some of the Spanish only walk the weekends, taking a few weekends to complete the last 100km.
5) Allow enough time to get your Compostela and know the process. Register ahead of time with personal information, go to Pilgrim office to get QR code when you arrive in Santiago and then you can go celebrate and take pictures at the Cathedral.