Train from CDG to Lyon - how much time to clear customs?

it seems like we can reschedule to the 11:57 free of charge

Just one thing to think about for your planning/decision-making -- change/cancellation fees may have been waived, but check if the fare difference will still apply (I just did a dummy booking for the same CDG-Lyon route and looked at some of the terms for various trains, and nearly all have no change fees but do state ajustement au tarif en vigueur, i.e. the fare difference will apply).

The fare difference will apply, if you reschedule, so you will basically pay the difference between what you bought and a same-day fare (which should be around 90€/person in 2nd class on that route). And trains do get full sometimes (though perhaps not midweek in November).
As I see that the 11.57 is indeed very cheap, that's what I would book. The fare difference will more than pay for breakfast at the airport while you wait, and for lunch or snacks on the train! In any case, I find that there is little value in getting to your destination before 2-3 PM, because hotel rooms / apartments typically aren't available before then, and it is nice to be able to nap a little after a long journey.

Our plane is scheduled to arrive at 07:50

Key word here is "scheduled". You could arrive a bit earlier, or a lot later.

On Aug 24 my wife and I arrived at CDG at 8:10am, and easily caught a 9:46am TGV (from Terminal 2). We had no check-in luggage, and were seated quite near the exit door of the plane. I posted the details in the forum: https://community.ricksteves.com/travel-forum/france/report-flew-into-france-cdg-aug-24-passe-sanitaire-experience
Before booking this 9:46am TGV, I checked the on-time arrival history of my flight (on flightaware.com), and liked the odds.

Took about 3 minutes to get through the passport check for me. It’s not a long walk to the train either. I got into Terminal 2A and it was a 10-15 minutes or so to the train station.

Thank you so much for the input, everyone!

It sounds like making the earlier train is not outside the realm of possibility, and since we'll be coming off an overnight flight with a four-year-old, I think we'll go ahead and book the earlier train. That way we can hope to make it to Lyon sooner (and maybe keep our fingers crossed that our room will be ready earlier).

@Jeff B - I've never used the flight aware site, so thanks for providing that, as well as the link to your trip report - super helpful!

Good luck with your train travel. Note: customs will be walking through a door labeled ” nothing to declare”. Passport control/ immigration is the important one.