I really hope your mid week journey goes smoothly.
To help things along, my advice is to cross the channel as early on the day as you can. If your booking is for the afternoon, change it to the morning if you can as the queues build up as the day goes on.
In addition to all the advice already given, I would also recommend having your phone plugged in and charging in the car during the drive there in order to have a full battery for any potential delay.
I was stuck in traffic on the M25 once and found it useful to listen to a radio play, it helped pass the time better than the regular radio stations. I sipped drinks little and often and managed not to need the loo urgently, thankfully. But I had stopped at a service station not long before, so maybe schedule a loo stop once you reach Kent and before you join the M20.
If you do get stuck for any length of time introduce yourself to your neighbours and explain that you need a nap, will they give you a knock if traffic starts to move.
Try to relax into it but fingers crossed you won't need any of these tips and you'll sail straight through with a car full of snacks!