And what you said, hiding the mirror behind the toilet door does not violate the requirements of installing mirrors in Feng Shui. Basically, you can only see the mirror in the toilet and close the door. This is also a collection. It doesn't disturb the feng shui in other places except the toilet, so there is no problem.
Another problem brought by the mirror is that people may be startled when they suddenly see the shadow in the mirror, which is also the reason why the mirror can't be placed directly in front of the door (feng shui reason). This is a psychological problem, so you should pay attention to it.