Roller shower blinds have been available for quite a while - we had one years ago at a previous house and it worked really well. Although it was white, it didn't mark at all in the 5 years for which we used it, although we were careful to wipe off any stray blobs of shower gel/shampoo and leave it down for a short while after showering so that it could dry/air off but it did just seem shrug off any water. Weighted rods at the bottom kept the blind hanging straight down and it didn't get sucked towards you like a shower curtain does. The rods were in three pieces - a longer one in the middle and two shorter ones at the ends so that the blind could follow the curve of the tub at the short ends. This arrangement meant that no water escaped the bath tub. It looked neat and unobtrusive when wound back into the cassette. Shower screens take much more looking after, in my experience, so I wouldn't choose to have another one. If I remember, our blind was relatively expensive compared to a shower curtain, but the looks, the ease of use and hardly any maintenance requirements made up for that. Out blind was still in pristine condition when we moved out. The next house had a separate glass shower cubicle - nightmare! Current new-ish home has a good quality, weighted shower curtain but will soon have a new shower blind. IMHO, you wouldn't regret getting one too.

Because the Mira round thermostat/ controller but is set into the tiling would it be easier and cheaper to look up the shower you have and replace like with like? It this it? https://www.plumbworld.co.uk/mira-excel-thermostatic-shower-19352-15218?msclkid=f4ee760811121c2d74982861331525c6&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping | Supplier | Mira&utm_term=4578229006415360&utm_content=Ad group #1

We did the same thing in our last house. Although we didn't use a tray, we created a 'step in' with a half-height wall with a bath screen on the top and just tiled the area rather than have a tray so the space felt more open. Here are some photos. At the widest part I think it was about 750. This allowed us to have a separate bath. The shower is tucked behind the door.

I think it completely depends on your unique family situation!!! Design wise, I prefer the rectangular tray that fills the space. However - will it realistically ever be used?!! I say that because mucky kids don’t stay kids for very long, and once they are smelly hormonal teenagers/young adults, they might not want to shower downstairs if their rooms are upstairs. (Mine wouldn’t!!). I think the idea of lovely built in storage as suggested by Angie is always a winner!! The other thing to think about though is future proofing your house for older age. If you think this is the forever home, a downstairs shower might be very handy indeed - however if the plumbing is in place, it could be hidden behind storage for a later time…