I live in Paris.

If you arrive at terminal 2, nothing better then the RER. Anyway, taking a bus, you will have to manage with your luggage the same way. After 9:00 in this direction, the rushhour will be over. train departs at 8:57, 9:03 and every 15 minutes after (9:12, 9:18....), and you are in Les Halles in 25 minutes. With taxi, Bus, hope there wil lbe no rain, otherwise....you'll be in trouble.

Passing the custom is easy if you do not have any luggage (for me, takes between 2 and 10 minutes at 8:30 arriving on Delta from US). If you have (seems it's your case), you will have to compete with other planes. Could take 30 minutes then.

If arriving in Terminal 1, that's a bad case, as you need a bus to go to the RER, and this is not comfortable. In this case, take the shuttle...if you find it.

For RER, the worse may be to get a ticket, could take between 1 minutes and 15 minutes depending on the queue. If terminal 1, bad case, if terminal 2, could be better.

Just remind to those who are not aware, you have the TGV (high speed train) in the same railway station, you can go to Lille (border with belgique) in 1 hour, to Lyon in 2 hours, to montpellier, avigonon, Marseille in 3h30 to 4h30. This is rather good for low rate (compared to Air France).