I would avoid taking the RER B during this strike, because it is extremely chaotic (check twitter). Very few trains, and you really don't want to be inside especially with luggages.

Le bus direct is a good alternative. Take into consideration potential traffic jam during peak hours (CDG to Paris between 06 AM to 10 AM / Paris to CDG between 4 PM to 8 PM).

Also, taxi will stay at fixed fare contrary to Uber that is doubling the prices (UberX 90€ Paris to CDG during peak time).

The best option can be booking a regular Taxi like G7, it should be less than 65€. Otherwise, le bus direct is OK. Montparnasse should not be affected by the protest of the 17th of december as far as I know, the protest is from Nation to Republique this day.

Allow plenty of time because of potential heavy traffic.