This might be something where you need to compare prices with and without the card. I just had a quick look at the French railway site for a ticket Paris-Lyon for 24 october (a weekday). With and without the card the fares were the same as far as i could see - first class ranged from €35 to €54 depending on departure time and then you can add things like lounge access and full flexibility for a few Euros more.
The Liberte card - which costs €399 to buy - gives discounts off the Business Premier fare. I didn't see those. But if they're like Eurostar they might be very high?
I have done as more playing as you suggest @MEL_Traveller and it appears that we are better picking train trips the day before or even on the day.
We are likely to have much better idea at that time and just wear the slightly higher cost booking at such short notice.
Thanks
Join Australian Frequent Flyer (AFF) for free and unlock insider tips, exclusive deals, and global meetups with 65,000+ frequent flyers.AFF members can also access our Frequent Flyer Training courses, and upgrade to Fast-track your way to expert traveller status and unlock even more exclusive discounts!