Kann mir jemand mit Elektrikerkenntnissen einen Rat geben?

My advice is: get an electrician.
You are not allowed to do this kind of work in Norway

If it burns down, the FIRST thing they check is the electrical system. When they find illegal work, the insurance might not pay out at all. If someone else gets hurt, you may go to prison

Even if everything goes well, it will be undocumented, and you will run into problems when the mandatory check of the electrical system comes up

If you ever sell the apartment, there will likely be a check on the electrical documentation. That leads to problems. Just get an electrician