To get to the Loire Valley, I would never pick up at CDG since it is in the opposite direction.
And on the way back, beware of rush hour traffic on the Paris ring roads.
When you head south, a good collection point in Paris is Gare de Lyon IMO. It is a very short drive to the quai de Bercy expressway leading to the Périphérique ringroad. Barely any city driving.
If you want to play it even safer, you could use Orly airport (access is now quick by metro line 14) or Massy TGV (access by RER B, very close to the A10 to the Loire). Massy would be my preference over Orly but there are more companies at Orly.
As for companies: I am not a fan of Enterprise, I am forced to use them for work and it seems that their locations are only staffed by interns and that their vehicle inventory has no slack whatsoever - my colleagues and I have driven off the lot with a white van instead of a car more than once...
In any cases their rates for the general public are rarely competitive so you are unlikely to end up with them.