We went through this kind of destructive behaviour with dd2, she is now 4.5 and for the most part has outgrown it. For her it was about testing her boundaries and getting my attention, she is the middle child. We quickly found out that rewards worked better than punishment. So she would get to go to the park at the end of the day or use play dough or bake cookies etc for each day that she hadn't misbehaved. I found that if she got one on one time she was much better. It sounds like you have the right approach with the gentle parenting.