So this train is completely sold out from Frankfurt (even for regular tickets). International routes are very popular and this one is no exception : all cross-border trains are sold out until 18:59. Weekends are even busier so it's no surprise that they're sold out.
However I've found a few alternatives for you :
Option 1 (close to your original plan) :
- ICE Frankfurt (Main) Hbf - Saarbrücken Hbf 10:56 - 13:00 optional reservation*
- stay onboard the same train
- ICE Saarbrücken Hbf - Paris-Est 13:02 - 14:51 a single 1st class seat left (30€)
*Sit on any untaken/unbooked seat. If you don't find any go to the dining car and enjoy a coffee.
Option 2 via Luxembourg :
- RE Frankfurt (Main) Hbf - Koblenz Hbf 09:08 - 10:54
- RE Koblenz Hbf - Luxembourg 11:06 - 13:23
- TGV Luxembourg - Paris-Est 14:10 - 16:31 6€
Option 3 via Offenburg :
- any ICE Frankfurt (Main) Hbf - Offenburg Hbf (optional free reservation through ÖBB)
- any RB Offenburg Hbf - Strasbourg
- TGV Strasbourg - Paris-Est 15:44 - 17:35 20€
Also an option if you're fine with leaving at 5:30-6am. Let me know