It's possible to do as there's a direct bus (number 91) that takes just 20 minutes (also possible to go with Metro / walking or change). But if you are unfamiliar with the situation in Paris I'd avoid the stress. 

There are also onward connections from Paris Est that can be reached by direct Metro. 

And there are also connections that avoid the station change in Paris, e.g. 7.47 from Rennes (arrives 13.51) or 12.00 from Rennes (arrives 18.21) or 16.03 from Rennes (arrives 21.51) (weekdays, slightly different weekends but of course you'd need to check your specific dates).

It’s a perfectly doable connection, but I agree. If you’re unfamiliar, I’d leave a little more time, or take one of the connections avoiding Paris. 

I think @Hektor is suggesting you change at Marne la Vallée-Chessy or Le Mans instead of Paris, which is very easy. You still have to change at Strasbourg, which I guess is the same as your original journey?

If you have any trouble finding these connections, just let us know your departure time and date.

Possible once a day.

The change is at Massy TGV, the airport CDG or it's a direct train to Strasbourg. But it depends on connection.