Why you would not visit the desert even in December? At night it is colder but days can be nice anyway. I have been there that time and the weather was nice. Many people are going to the desert just this time, between other to celebrate New Year's Eve in sand dunes.
Have warm clothes with you for the night, at day it can be enough with sweater or even a T-shirt. The best is to have a layer of clothes so you can take on or off when it needs.

From Ouarzazate to Meknes you can go by bus, Supratour. But on the way there are many places to visit, so plan it with overnight stop on the way like in Boumalne for to visit Dades or Tinerhir for Todra Gorge.
If you anyway not want to go to Erg Chebbi then you get off the bus in Errachidia where you need to overnight. Otherwise you continue with Supratour bus until Merzouga.
From Errachidia to Fes is overnight bus (it is the same bus you would take if you go from Merzouga to Fes).
But there are even local buses going during the day. Or you can take shared taxi, though it can be a little complicated because no the same shared taxi goes the whole way to Fes but you would need change to other taxi on the way.
When you arrive to Fes, take train to Meknes.

Your itinerary looks good. But I would maybe stay shorter in Marrakech and longer in Fes. You can see more around there like Meknes, Moulay Idriss, Volubilis also. And self Fes deserve more than one whole day only.
In Erg Chebbi you can stay one night at the hotel, one other night in the camp. 2 nights in the camp are too much while if you stay at hotel you can take a jeep trip around sand dunes, or go to Taouz and behind for to see old paintings in the rock. Near Merzouga it is also one lake, but not know if there is water since it was not rain a long time.
The best would be if you can rent a car and drive self, you would enjoy and see more in not so rushed tempo as you would have it with tour company. Cheaper it would be also than a personalised tour.

@timcullis , thank you for this, I feel comfortable now to drop Chefchauoen and add more days to the desert. We will do a private tour of the desert then travel by train/bus. I think our itinerary is now looking like this:
Marrakech 4 days
Desert 4 days
Fes 4 days - with day trip to high atlas or Meknes
Essaouira - 4 days - train/bus from Fes in 1 day
Marrakech- 2 days

Or I could drop a day from Fes or Essaouira and add it to one or the other?

hello
if u need any help from Marrakech to fez keep contact with me

No, there are no obstacles in the direction, travel time is the same.
By car from Merzouga to Fes it takes around 8 hours, so it is better to stay overnight on the way, for example Midelt is good place, even if there it is nothing to see. So you have then also time to make a stop in Azrou to look at wild monkeys and cedar forest.
From Merzouga to Marrakech it take approx 12 hours, but if you go by car you need to do at least one overnight stop. But because on the way are interesting places we recommend always to take 2 overnight stop.

I read that I can take an overnight bus from Fes to Merzouga, then from Merzouga just book a desert camp which by camel is only around an hour or two...

You need 4 days for your desert trip and you can do it step by step so to speak. On the way are many places worth to see, so you not need to sit on the bus 12 hours but divide it to at least 2 days travel. In Erg Chebbi you would stay 2 nights, 1 night in the room because bus arrive late so you get no time to go to sand dunes. And next night stay in the camp.
Then it depends of where you want to continue. If to Fes, take a overnight bus from Merzouga.
If you want go back to Marrakech, then it can be a little not really problem, but the bus is leaving early morning and you would need to go early from sand dunes and catch your bus.

On the bus is not allowed to smoke, but bus make stop so you get opportunity to smoke, between other like to eat, drink, toilet etc.I don't know how it is with to charge laptop, if CTM have this possibility, but Supratour don't. At least I not saw something there when I have traveled just by Supratour recently from Marrakech to Tinjdad. And it is just Supratour you need to take if you go to the desert.

If you have not yet, buy a guidebook, it is worth to do it and read what you can do and see on your way to Erg Chebbi.