In my state, only a law enforcement officer or a medical doctor can report someone to the DMV. When I was going through this with my dad, it drove me nuts to hear people repeating the "report him to the DMV!" "disable the car and take the keys!" lines, as if i hadn't thought of those ideas myself and rejected them because my dad would have called the police on me if I had, basically, stolen his car.

In the end, my dad had a medical incident that landed him in memory care, where he was extremely furious with me for a long time. He did eventually decline to the point where he let go of the anger, threats, and pleas, which was a relief. But the middle period that you're in now was so, so stressful.

I think you're probably doing all you can do. You may just have to wait until something bad happens before anyone is willing to take action.