TGV to Nice from CDG or Gare de Lyon

I think if there are more train options from Gare de Lyon and multiple transfers if you leave from CDG, leaving from Gare de Lyon is a good choice. If there is only one connection/transfer for the CDG train and it leaves within 3 hours of your arrival, I would take it. I try to limit how many times I have to move my luggage and a single transfer is really no different in terms of how much you have to move your luggage that going into Paris for a no-connection train. I assume that you are looking at trains that depart 3 hours after your ETA for trains from CDG and 4 hours after your ETA for trains from Paris. That being said, you could also consider flying to Nice since you are already at the airport.

It's pretty common to go into the city to catch a train since there are usually few options from the airport. In Rome you go into Roma Termini, in Paris you go to the station connecting to your destination, and so on.

Many posts on the forum ask how many hours to wait between the scheduled flight arrival time to their train. You don't know when you'll be able to get on the train, whether at CDG or Gare de Lyon. Flights may be delayed, passport control may be slow, etc.

For long-haul trains my advice is to decouple the flight from the train. Get a hotel near Gare de Lyon for the nite you arrive. Go to the hotel and drop your bags, wander around, visit a favorite museum, have a nice dinner. The next morning walk to the station and get on your chosen train, purchased at the low advance rate. No hours waiting in train stations because you had to pad the train departure time in case of delay. No worries about missing the train.

And the train to Nice is looong (because it isn't a fast train once it turns to run along the coast). It's a lovely ride, but it might be a bit much after a transatlantic flight and the time needed in between the flight and the train. I like the recommendation to give yourself an evening in between to give yourself a little break, get a shower and good night's sleep, etc.

After a long flight and since you'll already be at the airport, I would personally fly to Nice. As a self-transfer on separate tickets, I would allow min 4 hours between flights, but even then, you would arrive in Nice 4 hours before the train.
You can experience the train on the way back.

I agree with Balso. We have done this, and quite honestly, a long train ride would be exhausting after a long flight. Fly directly to Nice, and get a good nights sleep. Then you are at your destination, and can begin your exploring up and down the coast. Trains run everywhere, and you will enjoy your time on the ground. It's an easy flight.

Thank you all for your excellent advice--much appreciated!