Chiseling out an opening in a Ytong wall afterwards

Hello to all craftsmen, I have a free-standing Yton wall as a room divider, it is 17 cm thick and only 120 cm high. Now my problem is that the heating engineer and sanitary professional came and installed a Geberit flush tank with a drain pipe in front of the wall. He said I should cover it, but then the wall will ultimately be 30 cm thick. That looks ugly. Is it possible to retroactively make a niche for the tank and the drain pipe in the wall? Then I can at least minimize the thickness. Thanks in advance Monika

Remove the "old" wall and rebuild everything completely. Should be easier.

We had a cat flap cut into the Ytong exterior wall afterward. With the chainsaw, it was a matter of 5 minutes. How big does the hole need to be?