ᐅ Cutting out openings in Ytong walls after construction

Hello to all craftsmen, I have a free-standing Ytong wall used as a room divider. It is 17 cm (7 inches) thick and only 120 cm (47 inches) high. Now, my problem is that the heating and plumbing specialist came and installed a Geberit flush tank with a drain pipe in front of the wall. He suggested I should cover it, but then the wall would ultimately be 30 cm (12 inches) thick. That looks unattractive. Is it possible to later create a recess in the wall for the tank and the drain pipe? That way, I could at least minimize the thickness. Thank you in advance.
Monika

Remove the "old" wall and rebuild everything from scratch. That should be easier.

We had a cat flap cut into the Ytong exterior wall afterwards. Using a chainsaw, it only took about 5 minutes. How large should the opening be?