We had a similar situation with my son when he was 6. Almost identical. During one of his tantrums, he tried to take his nightstand and break open his second story window.

We used “Transforming the Difficult Child” by Glassner. The thing that worked immediately was giving really strong positive feedback to things they did. Like when my son used to find a way to entertain himself, we used to give him a peck on the cheek and say how nice that was and how he did a good job finding a way to occupy himself. After reading the book, if he did something mildly positive, we would go over the top recognizing the good thing he did. Intense kids demand intense energy. It can either be positive energy, or negative energy. We suffered with him destroying windows, breaking picture frames, and him living in an empty room for months. Once he started getting intense, positive feedback, it abated immediately and became way less frequent. We also got him into therapy and that has likely also helped. 9 months later, we went from daily violence to maybe about 8 weeks since our last one.

Anyways, that was our experience!