This trip is planned as a self-drive trip as Oman is a big country and you need 2-3 weeks hence you can start it from Muscat.
Head to the Sur from Muscat for your third and fourth day. It is also a beautiful place to spend 2 days and explore Oman.
You are exploring Oman and if you do not give your days to explore Desert then it would be a total waste.
This will be the longest journey for your driving tour but it is the best place to get to know Oman in details.
After exploring the desert in Oman for two days, you should give your last 2 days to cool down in the Mountain.