Trying to find a technical solution on the aircraft to the “problem” of a suicidal pilot is pointless, IMHO. Whether it involves doors, other people, computers or whatever, if the guy trying to crash the aeroplane and kill himself and possibly others is determined enough, they will succeed despite any precautions.
There have been many fatal accidents in the past where the pilot(s) have been doing everything in their power NOT to crash, yet it has still happened. I think most people who don’t fly for a living truly don’t understand how a moment’s inattention or an incorrect control input during a critical phase of flight (like takeoff and landing) can lead very quickly to disaster. A deliberate contrary action could speed that up to almost instantaneous.
For non-pilots, imagine sitting in the passenger seat of a car doing 60mph. Just as you are about to pass a car going in the opposite direction at the same speed, the driver turns sharply into the path of the oncoming vehicle. Even if you had a duplicate steering wheel fitted on your side, it would be too late to do anything in the fraction of a second that remained.
The real problem is how to avoid having a pilot with severe mental health issues operating an aircraft. By the time they are sat in the seat it is too late...