Hi, probably too late to comment now as you've already gone with someone on this project. Along the wall is the most common solution as it is easier to create falls, particularly if you have large format tiles. You will get water out of the shower given this is a just a freestanding panel, and it will also be cold in winter (speaking from experience) without a door on it. In the Australian Standards you are required to have a 'water stop' or step down to at least 5mm - most people don't do this and this is where problems can start from a waterproofing point of view. Good luck - hope it goes well

Ok first, the 750 wide bath gives 250 more wall space between shower and bath to allow a 750 wide towel rail there allowing for bath sheet size towels. For me this is the most important. Depending on how many towel rails you need you could consider hanging them on walls at back or side of bath too. This addresses functionality first. Opt C out. Niches. Cutting and tiling a niche hole in a brick wall is expensive compared to framing a little height change at end of bath. Niches are also harder to clean and will offer less shelf space. But will add width to area not reduce it. I don’t see an issue with messiness having it on same wall as shower niche. It’s your choice. A fuller floor plan may also help people commenting. Good luck

This with the extra width you suggested the built in bath with a large shower space next to, the shower head coming off the opposite wall to help avoid water to window, the window would be best tiled, the wall with the shower door into "The wet room could be all glass up to the vanity. There are several suitable places for towel hanging and an extra storage cabinet could be above the toilet, I forgot to add this

Look for a piece of carpet and get it edged. Ask at carpet shops to see remnants or off a new roll