Advice from a structural enigneer, to give your home the best chance of being stable and crack-free ensure the drainage around your house is pristine from day one. This is really out of your hands. Your only power is to be super-annoying to ensure your builder follows our guide on-site drainage for a waffle slab house:

No, builder coming around tomorrow for first time, as is Class 2 designer (been a rush to get a bunch of class 2 consultants on board for a simple reno). So far all my own investigations. The slab is structural, 275 thick with localised thickened beams over the posts in the carpark. Opening it up would probably be refused by Strata. I can go look at the original structural details to see how much cover is over the reo - maybe I can get a 20-30mm scabble and do the rest with screed to cover before tiles. It'd be very flat though. Will need a structural engineer to sign off on the detail, and if I'm honest, I'm uncomfortable reducing cover to the reinforcement as below and think it opens up a bunch of risks. I'll run this past them tomorrow for their comment though.

I won’t comment on the plastic because it’s not my area. But a lot of new builds require a concrete pathway/strip around the perimeter of the house to ensure the building warranty. Check your documentation or ask your builder. Your plan to butt grass up to the slab may impact your warranty.

Yes but smaller pieces still. Like 1m x 1m. Tried to get my offcuts from tradies but they refused to onsell, unless they made something out of it for us.