I've built 2 x houses, carports and a couple of big sheds since being here.
I put reo in everything as it is cheap, takes little effort to incorporate and increases durability.
I thought you were going to put a hot tub on it, do some research and find out how much it weights when full of water and people,
I'd reinforce it and the steel mesh is as cheap as chips.
You would be better off putting a 2.4m x 2.4m x 100mm high shuttering ply timber surround around the area to be concreted.
I would get 2 x 2.4m x 1.2m steel reinforcing grids as well, make sure what you have there is compacted and then get one of those 'Concrete Taxi' type firms around with...