Car rental return at 0600 Monday morning at Charles de Gaulle

If you need to be at the airport that early, my advice is to return the car the night before and spend the night at one of the hotels at the CDG Terminal - there are several, including Ibis, Novotel, Hilton, and Sheraton. There are just too many possibilities for delays trying to return a car before an early morning flight and you do need at least 3 hours prior to a US bound flight. There is an underground shuttle train connecting the terminals, which is accessible from the hotels.

Here is a cut n paste from the Hertz website-

-"""-From 0400 to 0600, only terminal 2F is open. Terminal 2G is open from 0700 to 2200. Desk for pick up and return is at the car park outside terminal 2G""

https://www.hertz.com/rentacar/location/france/paris/CDGT50

You really should be asking Hertz where they want you to drop off the car.

And Carolyn's suggestion to 'drop off the night before' is what I think is a great idea

I'd drop mid afternoon the day before, get a hotel AT not NEAR the airport and then take the train into Paris for dinner and evening stroll before returning to the airport for your early flight. Car rental services are very cumbersome in Europe; we have rarely gotten a car in less than an hour and it took us 2 hours at FCO; drop offs are also cumbersome if you require a receipt showing the car undamaged and gassed. In one ludicrous situation it took us 3 hours to get copies of paperwork showing extensive pre-existing damage on the car we were returning. We had for reasons complex had to accept a badly damaged car and so documented it very well; in Paris they insisted we surrender the paperwork and we insisted we have a copy so we could fight any attempt to stick us with the repairs. In a dazzling display of passive aggression and countless 'broken' copy machines, it took us 3 hours to get that material. The personnel clearly thought we had a plane to catch and so could be outwaited. We didn't and weren't. Be sure to keep your receipt from your gas up (we had to prove we were gassed up last fall and felt lucky to have the receipt) and don't put yourself in a rush situation.

And what if you have a fender bender or worse on the way, or there is some hideous wreck involving a couple of tankers and the road is blocked for a couple of hours, or you get a flat tire? Never make getting to a high stakes connection dependent on perfection.

Thanks so much for your advice.
I will return car on Sunday.....not worth the hassle of Monday morning.
Will just now need to rethink our plans for Sunday....