I dont think you did anything wrong. You want another respond but were disappointed.
I owned gsd in the passed and one of them was very submissive. When poeple approched he pied and lay on his back. In the fallowing months that changed thank god. He didnt lay doen anymore but wasnt protective at all.
When he became 1.5 years old he became the most steady dog i ever owned. Only protective when things were wrong. Very dominant but not agressive, pure bodylanguage. Very pleasant dog. Ofcourse i know a cao is diffrent but you must be patient. My first co was protective from day one but not that stable i wanted. Oh he would kill you because when he didnt feel secure he made the choise to attack and never backed down. My second became to bark when he was a year old.
So give him time to mature, the character will change a lot