easy answer you can´t look before how many seats left :/
to save reservation fee´s i recommend to take the afternoon TGV from Frankfurt directly to Marseille (can be reserved at a DB Reisezentrum)  this way you have to pay for only one reservation instead of 2 and you save the expensive Thalys reservation :)

Option B take the ICE to Brussels and catch one of the few TGV´s from Brussels to South France (Marseille,Nice,Perpignan)

I don't know if you meant it that way, but unfortunately there is no longer a direct TGV from Brussels to Nice and Perpignan?

There are currently 2 direct connections per day to Marseille St Charles, and 2 to Montpellier St Roch.
And also 1 direct connection to Montpellier Sud de France.

ahh good to know one of the Marseille usually went further to Nice and one of the Montpellier trains to Perpignan in the last years :/

As iam avoiding to use Railpasses in  France and Spain as much i can i am not always up to date on the Route changes there :D