You probably know that SNCF offers various discount cards, yesterday they sent me an email update and it appears that you can also save on fares outside France, as well as onboard discounts and Avis specials. You will likely need to drill down into the terms and conditions to see what is offered. It's a bit complicated but could save you some serious euros.
https://www.sncf.com/en/passenger-offer/mainline-cards-fares/avantage-adulte-card