Order the concrete ready mixed route ....
much easier .... just prepare your area with the timber boxing, re-bar and that's it.
12cm thick is an overkill for parking, I would do 10cm/100mm
7x3.0.10 = 2.1cbm
order 2.2 cubic meters by truck.
If you do it yourself, just to save a couple of thousand baht max, and all that work with bags, sand, gravel, mixing ..... no thanks.