People who advise, on this forum, to give yourself a 3 hour margin when flying home out of CDG, know what they are talking about. We did that last fall; and I was surprised to notice, when we sat down at the gate to await boarding, that 2 hours and 20 minutes had elapsed since we walked out of our hotel’s front door (Sheraton, right in T 2). You not only have the usual check-in, check bags, security clearing to deal with; but there is an extra step of registering that you are leaving the Schengen zone. These are not necessarily near each other, and it’s a big airport. Just walking from one place to another took some time, and there was a certain amount of line waiting. So, 3 hours pre-flight plus an hour if you take the RER, puts you at 6:30. Presumably, you’ll need some time to get ready, as well. 6 or earlier wake up. Myself, I would rather have the extra hour of sleep gained by staying at the airport, as well as the serene feeling of knowing that no accident or other unanticipated event will cause me to miss my flight.
There are many options just outside the end of the tram line that runs between terminals, if you don’t want to pop for the Sheraton. You could always have your last night in town and take the RER out after doing what you want to do.