Thanks everyone! Perhaps leaving it is the best option, it is certainly the easiest. I suppose the new owners can change it, and it won't be a deal breaker to sell the house. I think it bothers me because I have a touch of OCD : ). In case anyone is interested, this is what the kitchen looked like before and after:

Hi, unless you take a bath at least once a week I'd skip the tub. I'd rather enjoy a bigger walk in shower, which is also a lot safer than climbing into a tub to shower. If you have the space instead of a tub and shower I'd consider a double vanity to give you extra storage space. It's a little luxury you will come to enjoy when everyone is getting ready on the morning.

I had a other look at this post because the subject of a laundry trough often comes up. They are required but the construction you’re putting in will meet the requirements. Below are some handy references. The video (presenter isn’t going to win a Grammy) is quite good. National construction code link. https://ncc.abcb.gov.au Video from abcb (Australian Building Code Board) dealing with the laundry tub issue. https://www.abcb.gov.au/resources/videos/i-dont-want-laundry-tub-can-i-use-sink-instead

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.