That's a really long drive you're attempting there. Yes I think going via Reims (nice town) is the most direct route but it will be at least a 7 hour drive from Munich assuming perfect conditions and no traffic jams (a big assumption!). Bear in mind December is getting pretty cold and the roads could be dodgy.
Also, for the second day which isn't so long you have to contend with crossing the channel. Are you going to put the car on the train or a ferry?
It's certainly not feasible to do the whole trip in 1 day and I would recommend 3 days. Then you could stay in Strasbourg (gorgeous city) and maybe Lille.
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