I live in the country and am completely reliant on tank water. I also have a grey water recycling system and can tell how much water has gone through it at any one time as the filters need to be cleaned after so many litres of water go through them. I can almost guarantee that the bath with a couple of grand kids splashing about for twenty minutes having fun will use less water than any of their parents showering does. Even topping it up with more hot water if needed. Unless you are very conservative with your showers baths are not that bad and the value they add to the resale of the average home outweighs any downside. I definately wouldn't buy a house without one. I have had a house that has a bath the size of the shower though, about 1m x 1m by maybe 40cm deep and I think it was called a hip bath. My sister in law has one at her house, she got it from Lanark, as her house only had a shower which is not practical and it was the cheaper option than redoing the whole bathroom. I dont know if that is what you are after or not.

by "other options" i mean that you should consider all other options for locations of fixtures..there is already plumbing along each wall so swapping the location of shower and bath or vanity isn't as challenging as if there were no original plumbing in that area and if you plan to replace the fixtures and tiles rearranging the fixtures will only be a small percentage of the budget and it's a shame to limit the final result without at least considering other options..and although there is enough space to have the vanity hard up against the wall my suggestion is to have the vanity away from the wall to make it easier to clean and also because the small nook is an ideal space for hanging a hair dryer and with the luxury of so much space it will be easier to install a vanity away from the wall

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.

The gap is definitely driving me crazy, we’ve only moved in since March so been lots of pondering, including changing it all, but unfortunately the next room over is our third bedroom. But you have given me an idea there is a cupboard in the front of this bathroom and could knock down the wall so maybe the bathtub could actually get a bigger spot after all and get rid of the front cupboard / wall and then change where the door is.