Hi Bob
I'm not an expert but I have travelled in and out of CDG a few times. So I offer the information below. But, .if you can give yourself another day, you can take the pressure off by staying overnight in Biarritz or Bayonne and making your way to SJPP the next day, as
@Stephan the Painter suggests. It's a short one hour train ride from Bayonne to SJPP.
Safe travels whatever you decide.
In theory, you can make it in time for your flight to Biarritz. But it would require everything going quickly and smoothly in one of the world's busiest airports. That depends on a range of factors including how many flights arrive at the same time as yours. And whether you need to change terminals to take your flight to Biarritz. That can involve a long walk and sometimes a train shuttle.
See info further down the page regarding terminals.
Back up plan. If it were me, I would have back up plan, as you've mentioned, and it would likely be the 14.05 from Montparnasse, rather than the 12.11 just to allow for delays. Or, even better, if you can take the train from CDG train station to Bayonne. There are often a few trains a day that don't require you to go into Paris Montparnasse.
One thing to note - there are Rugby World Cup matches in Bordeaux on 16 and 17 September. Trains going to/via Bordeaux in the days leading up to those matches will be heavily booked. So if you are going to book a train, I'd do it as soon as you can.
CDG Airport information
If the terminals are not shown on your tickets, this link will take you to a page where you can search which terminal you will arrive in and depart from. If you're lucky, you might be within the same terminal so that will be quicker. I've attached a simple map of the terminals.
Which terminal for your flight (Arrivals and Departures). Airlines list at Paris-Charles de Gaulle Airport (CDG 1, 2, 3).
easycdg.com
The Easy CDG website gives information about Arrivals procedures. I've included the link below as well as an extract relevant to your situation, i.e. your flights are on separate tickets. My understanding is that will mean you need to go through passport control and security as if Paris were your final destination.
Arrival Procedure Guide for Passengers landing at Paris-Charles de Gaulle Airport.
easycdg.com
See this below from the Easy CDG website. It refers to your situation, i.e. separate tickets.
■ If you are travelling without an official flight connection (you bought two separate tickets for your flights and need to re-check at CDG)
You may need to travel between terminals outside of the transit zone.
In that case, you should allow plenty of extra time to exit, collect your baggage from the carousel, re-check in with your airline and clear security.
Please ensure that you comply with
entry & visa requirement.