Train from Paris to Saint-Remy-de-Provence and rent a car

Usually you can book train tickets as far as 6 months in advance, so you should be able to see all the routes for your July trip now. Yes, you should prebook everything. Go online and plug in your dates and it will show you all your options. You can definitely catch a train from CDG to Avignon. It takes between 3-4 hours depending on how many stops. There will be car rental places near the train station.

I am doing a similar trip in May. We arrive at CDG at 06:00 on a direct flight. I have booked a TGV ticket on a train direct to Avignon, departing from CDG at 8:30. I used the SNCF app, but you can use their website, or Trainline.com to book. There is a very large car rental park directly across the road from the Avignon TGV station, with all of the large companies represented. I booked a car for pickup there, using Autoeurope.com as the broker. Depending on which terminal your flight arrives at, it is either a free shuttle train ride or a longish (but indoor) walk from passport control to the SNCF-TGV train station- everything is well connected. The station name on the SNCF website is Aeroport CDG 2 TGV Roissy. The two and a half hours I have allowed to get to the train will be plenty if the flight is on time, but I paid an extra €8.50 insurance to allow for last minute changes if I am delayed. There are lots trains from CDG to Avignon direct, which take between 2:50 and 3:30, so you should be able to find one that meets your comfort level. It’s about 21 km from the Avignon TGV station to Saint-Remy, and the station is outside of the city, so you don’t have to negotiate city streets as soon as you arrive. Bon voyage!
Edit: Make sure you enter Avignon TGV as your destination when booking your train tickets, not just Avignon.

What train station in Paris airport to go?
The train station at CDG and if there are no trains leaving from CDG, then proceed to Paris and leave from Gare de Lyon.
How to get there?
If you are traveling from CDG depending on your arrival terminal you either walk to the station or take the CDGVAL, the free inter-terminal transportation ssystem at the airport. The train station is in Terminal 2. If you are going to Paris for the train, follow the signs to the official taxi stand and take a fixed price taxi to Gare de Lyon. The fare is currently 55€ for up to 4 people.
Where to buy tickets?
I'd buy from trainline.com --it's more user friendly than the SCNF site, I have a SNCF account and sometimes I get so frustrated with their system that I buy from trainline.
In advance?
It depends. Allow 3 hours between your ETA and the train's scheduled departure time if you are taking a train from CDG and 4 hours if you are taking the train from Paris. The cheapest advance tickets may have restrictions on your ability to change your plans.
How long it takes to get to Avignon?
Approximately 3 hours from CDG and about 2.5 hours from Gare de Lyon.
What time we need to schedule rental car that day?
Based on your ETA, and assuming you take the 12:37 from CDG, I'd say 3:30 PM/

Thank you everyone for this information. Big help!