Shell construction 146 cm Shower 80x80 there remain about 60 cm next to the shower for the toilet .. build yourself a niche like that next to a cupboard and then try to use it as a toilet, wiping your butt afterwards.
I was able to fit a 1x1m shower into our 2.8m2 bathroom thanks to a sliding door, maybe that's an idea for you.
Sounds interesting. However, I can't really picture it. Do you perhaps have a drawing or a photo?
We were thinking of a 75*75 shower, for example, so really small. Well, maybe you're right and it doesn't fit. We imagined it as a fallback option for the kids or guests.
.. .unfortunately your drawings are not dimensioned and there is a huge difference between 2.5 m² and 4.5 m².
... without a floor plan your comment is unfortunately not helpful at all. 2.8 m² could also be 1.35x2.05 .. and depending on where the door is, I can still imagine a shower there.