I'm just wondering if it's too soon for all September SCNF trains to be showing availability? In June there are many more trains from CDG to Bayonne then in mid September. Any ideas? Or maybe they're all sold out already?
Your problem is that you are leaving from the airport. Look at the itineraries in June. They are longer and all require 1 or 2 changes.
In September, you have many direct 4- hour TGVs from the Gare Montparnasse. So you should inquire about booking your ticket from Montparnasse and not the airport. You can take a taxi door to door from the airport to the station or take the RER and metro.
And yes, some trains could possibly be added but they will be longer and require changes.
Thanks so much for this information! It really helps!