They are currently being built with limestone sand. Is that necessary?
Not necessary, but fine that way.
I also don’t know what to do with the office/guest room. The window recess is out of place... and I’m also missing a wardrobe or a spot for a wardrobe piece of furniture.
All in all: urgent restart, when apparently so close to the goal it still pinches in dozens of places and as soon as you try to improve it, it pinches somewhere else.
In our house (70s) [...] it was solved in such a way that the exterior wall at that spot was only built with half a brick, so the pipe could disappear there. Maybe that’s a solution.
That was a solution in the 70s, which is practically prevented today by the currently valid energy saving ordinance.
If you use drywall instead of limestone, you can certainly run the pipes inside the wall. Of course, it depends on what is above/below.
I already suggested something similar here, where possible – in my view, only between child 2 and the children’s bathroom.
However, I find your tossing back and forth of the floor plan at this advanced planning stage borderline.
There is a sketch by Loriot where a visitor waits in a room where everything is in perfect order except for a slightly crooked picture. The visitor tries to straighten the picture and at the end of a domino effect, the room is completely wrecked. That’s the model the OP has taken: take a design for a cookie-cutter villa (no burner, but works), then start pushing a single wall on the ground floor basically just because of a few centimeters until eventually a somewhat oversized bathroom is created upstairs. Instead of simply placing the "shower" object redundantly according to need, divide the bathroom and – at least as a nice social game for the forum – make a doctoral thesis out of the most original partition wall layout. Voila, here’s the dead end!