Growing some green grass.
So why remove 200mm of topsoil?

That's valuable stuff. And the use of "environment friendly cardboard boxes"?

There are many weeds that flourish in shade, as there are grasses. Even if they are covered, if they have had a chance to seed, they will grow. Grasses such as Kikuyu have deep roots, and if you want to get rid of it for good, you will struggle.
If you want to kill everything and start afresh, then use a non-discriminatory herbicide like
Glyphosate, it's cheap and effective, kills everything that is green. You can then either burn or remove the dried grass / weeds, loosen the soil and plant the grass of your choice. If it's kikuyu, you will need more than one application, in Winter, because Spring is far too late.