Replace a standard washbasin with a washstand system myself?

I haven't read the entire thread, but I can still recommend asking your kitchen dealer about the washbasins; they also install everything and provide an installation sketch (like when buying a kitchen), so that the sanitary connections can be laid accordingly right away. That was our alternative to the standard from the general contractor.

Kitchen dealer for a washbasin in the bathroom? That's new to me :-D

Kitchen dealer for a washbasin in the bathroom? That’s new to me :-D

Yes, we ordered all our bathroom furniture from our kitchen studio. For example, Nobilia also offers bathroom furniture.

Afterwards, just like with the kitchen, we received an installation plan, which we handed over to our plumber. The assembly was also done by the kitchen studio. It went really great!

So with our detached house, it is also the case that the sanitary ceramics cannot be removed. The washbasins are always wall-mounted, so that you can relatively easily install your own cabinet underneath afterwards. If I understand correctly, you want a combination of above-counter washbasin + cabinet, right? Which washbasin / cabinet model do you want exactly?