The only issue I see is the very unlikely event that you don’t get the 30 day extension. It’s at the discretion of the immigration office. But, as I said, it’s very unlikely.
You don’t need to make the trip to Cambodia. You will get 60 days visa exemption on arrival and can extend it by 30 days at an immigration office.
Also be aware that the airline can ask to see your exit ticket out of Thailand and it has to be within the first 60 days. Immigration, of course, have the same right.
If the car you are planning to drive is a rental car, you may find there are restrictions about taking it out of the country.