Is a second shower bath sensible?

Excuse me? 4 sqm of tiles for €1800 or does that include the [Vorwände] etc.?

with four sisters I imagine that is also more difficult when I think about how long I sometimes need in the bathroom.

With us, one always leaves the house while the others are still sleeping, then the other parent & the child follow. That probably won't change unless we are both at home.
We have overnight guests 2-3 times a year.

only the tiling work. So for floor and walls. According to the description:
"Shower area, tile height 2.00 m above shower tray, area approx. 4.00 m²
Area behind WC, tile height 1.60 m, area approx. 1.80 m²
Wall at washbasin, tile height 1.60 m, area approx. 4.00 m²
Floor area 3.70 m²"

How much does it cost to just install water and fans in preparation, that is, 1st expansion stage? I would then have the expansion done later by a cheaper company as needed. Maybe you will move upstairs yourselves. Also, the RH can be sold better then, because the living value under the roof is higher.

Difficult, as dead branch lines are not allowed to be laid - costly to separate them in the connection room afterward.

That is why I would already create the separation, a simple toilet and a simple washbasin on the wall. If in doubt, also a simple shower tray, although with clever planning you can possibly prepare everything (in the floor and in the wall, laying the pipes up to the washbasin, but not connecting them there. Inspection hatch in the wall or something like that.

ypg, level 1 is at €1600 and level 2 would be €3200

At first, we definitely wanted to have at least the wiring installed, then we thought about it a bit and asked ourselves how we would then get electricity and light into our constructed walls.
Possibly, the developer could also have the electrical and lighting wiring installed? The cables would then not be visible but in a duct.
Possibly have the walls made by a company as well as the electrical/lighting wiring at the same time? But this point directly at move-in, so that you don’t have to change the rooms upstairs later on. This way, at least the wall would already be there.

We had already planned level 1 and 2 and also a buffer, but is it worth putting "all" of the buffer into a bathroom that we will hardly use at all in the first 5 years? There are many other things that one would use immediately.