The company we've been using has front load and rear load trucks. They all hold 10 yards, but they only load the rear loaders to about 7, due to weight restrictions, or something like that. At the shop, the floors are 4", no rebar or mesh, just fiber in the concrete. There has been pretty much no uncontrolled cracking in the 11 years that I've been there! I like the gravel idea. You could leave it a little bit low, then if you decide you want it gone, just get some topsoil put on top of it.