Getting to Mont St Michele from CDG

I would just spend that night on MSM. Rennes is so close might as well just get there. Trains (maybe buses) can be used. Might be harder depending on the day of the week. Just google it.

This is a time when Rome to Rio is a good starting place. Train seems to be the most practical way.

Be sure to do your research. The train from CDG to Paris arrives at a different rail station than the one from which train towards MSM leaves from. You didn't mention car or train, however. Driving a car can be dangerous after a night with little sleep. Are you continuing around Normandy or Brittany after MSM?

We only spent an afternoon at MSM, so I can't comment, but posters here have reported that the few hotel rooms on the island itself are small and hard to book. You might wish to consider the soulless modern hotels on the "strip" of tourism services within 1/4 mile of MSM itself, if being close to the island is important to you. I just mean to increase your menu of prices and choices.

You can take a direct TGV train from CDG to Rennes (allow 3 hours from Plane landing time to Train departure time), of which there are at least 3/day, followed by a bus from Rennes to Mt St Michel (info here: https://keolis-armor.com/45-Schedules-26-prices.html).
If times from CDG don't suit, you can take a train from Paris-Montparnasse to Rennes, in which case you need to allow 4 hours from plane to train.
If whatever you do, you arrive in Rennes after the last bus, then there are hotels right at the train station. Ibis is OK and cheap, Le Saint Antoine (Best Western) is nicer but pricier. Try to email them and ask them if they'd keep your luggage for a day (even for a small fee) until you get back from Mont Saint Michel. There is no left luggage facility at Rennes station alas.

Thank you, I appreciate all the great responses.