Hi I think in reality and once it is completed it will be a little too small. Often plans don't show that. A square semi recessed basin as you have shown will be around 400mm deep, plus a bit of cabinetry to the rear say another 50mm. That leaves your main walkway and access to the toilet/shower reduced to 650mm. The shower screen will also need to be around 600mm long to be effective for water containment and that leaves an opening of only 500mm to get in and out which is too small. We have done smaller screens as you can see below but the fall needs to be spot on with the tiles. You can overcome the small walkway by making it an open ie visible to the room ensuite and access is easier. This is one we did many years ago as one of our first reno's. Cheers

based on the accurate dimensions the most economical update would be to replace the spa with a shower over bath, keep the existing vanity and toilet, reuse existing taps and retile but, any bath renovation is always much more expensive than imagined, and the unexpected expenses (eg asbestos under tiles) can add up and a minimum of ten thousand for a very modest upgrade wouldn't be surprising...If the bathroom is in acceptable condition especially if there's any possibility of a future expansion towards the backyard that could affect the current position of bathroom and laundry) i suggest (subject to confirmation from a plumber) that you consider reorganising the other spaces and sacrifice the wiw to create an ensuite with linen storage taken off the middle bedroom..surprisingly an extra bathroom could be less expensive than renovating the old one but i would still allow at least $10,000 with careful planning

I thought pebbleCrete was dated . I would not use it it’s hard to clean and it’s not clean look you want. Why not save money and do it properly. In the meantime just put up with concrete.

Yes,I had a similar sized spa in my current house. You couldn't get more than two people in it. It was too deep for toddlers. It was another thing to keep clean - as the automatic pool sweep doesn't clean it so it has to be done manually. I used it once or twice over five years and ended up getting a plumber to disconnect all the pipework. I converted it into a garden bed with agaves who love it! Go big now or you will hate it for ever (unless you fill it with soil and agaves). Best of luck, Dr Retro of Dr Retro House Calls