TGV Paris to Bordeaux

I suggest you read this webpage: https://www.seat61.com/international-trains/trains-from-Paris.htm
If you scroll down you will see a section "Paris to Nice, Lyon, Bordeaux, Lourdes, Chamonix & other French destinations from €20..." which should answer your questions.

You can use Ouigo low cost high speed rail instead the tickets are a lot cheaper than TGV
https://www.ouigo.com/

I took Ouigo from Bordeaux to Paris a week ago. Just bring your own eat, drink and entertainment

"Has anyone done this?" Yes, a few years ago. Cost varies depending on when you book. What kind of suggestions are you looking for? Trip took around 3 hours and we bought non-exchangeable first class tickets for around 25€.

Since last year's upgrades, the direct trains now take only 2 hours. Full fare is a bit higher than Paris-Nice, but this reply that I wrote elsewhere today still applies:

The one way ticket price range in second class can start from 30 or 40 euros on a limited number of seats, which are the first to sell out, and there are intermediate prices, and full fare is over 100 euros per person. If your date is not yet for sale, you can request email notification through a test search at https://en.oui.sncf/en/.

SNCF running sales on spring tickets from €25 Paris to many destinations. Check it out!

To offer a little explanation, the TGV service on SNCF has been rebranded Oui.sncf ; see https://en.oui.sncf/en/

There's a bus system as well as the various high-speed and regional train services available, along with the specials of the season.
SNCF is the operating company.

Clear? No, not for me either; I had just finally mastered the older designations.

I've done it a couple of times. It took 3 1/4 hours, and that seemed very quick. Now that it's down to 2 hours, it's amazing. As others have suggested, book your tickets as soon as they go on sale for the best rate.

When do the tickets go on sale ?

120 days out or 92 days ?

Yesterday we just bought TGV tickets for May 17th CDG- Avignon so more than 90 days out for sure