What is the religious ruling and the optimal course of action regarding a wife who has removed her hijab after wearing it, knowing that she prays and promised to wear it before the Hajj?

What you are doing by advising your wife to wear the hijab is part of fulfilling your role as her maintainer (qawamah). The wife must understand that the hijab is an obligation upon her, and obeying her husband in what is right is mandatory. She must not display her adornment to anyone other than those for whom Allah has made it lawful. Continue to advise and admonish her. If she persists (in not complying), then forsake her in bed. If that does not work, then strike her with a non-injurious strike, and try to keep her away from negative influences.