MS9 was my second ever route when I did it 3 years ago it was nice and easy, some development on the edge of Ouarazazate made that section of the route a little out of date. Theres a section along a river bed that may have had some weather damage since I went that way.

Given the timing of your flight, clearing the airport etc you've got a 4ish hour drive to Oarazazate so take the N9 straight there. Generally I'd avoid driving at night as much as possible, done it a few times and it can be alarming. Once you get down from the mountains the road into town is decent. You could look at accommodation around Ait Ben Haddou which is worth a visit although touristy. Its closer to Marakech possibly 3.5 hours drive. I've stayed at Kasbah Cigogne which is nice.

My original idea via Taroudannt wouldn't fit easily with your flight, the route is quite a bit slower.

What do you want to see/achieve during your week? Obviously some off road from your original post and as a first trip possibly some of the sights and Essaouira. It would help advise/suggest routes.

How will you be navigating? maps or gps?

Given you flight time I'm also thinking reversing the route might be better. The road is very good between Marrakech and Essaouira and you could easily be there and having dinner in the Medina on your first night