Start relaxed with the door and the window. Once installed and the window is tightly mortared, you can also walk into the garden over pallets for a few days.
Instead of concrete and iron, you can also use mixed recycling. Or as mentioned, simply point foundations. In my opinion, your variants somewhat overshoot the mark. Please dispose of the asphalt and no experiments. That doesn't necessarily cost a fortune. Remove soil. 25cm recycling, 5cm chippings + slab thickness later.
By the way, I imagine a round terrace with terrace slabs and shuttering stones to be difficult. There will definitely be thick gaps. 9/11s are more suitable there.