Not all children are wired for time outs, although that's the most recommended discipline method. Are there privileges that can be deprived? You mentioned hitting behavior, if sent to her room without privileges, what would happen?
My oldest, didn't respond to this method nor to a rewards system, for good behavior. He tends to get himself into an emotional state of mind, that once in it, it's as though he cannot get past the elevated emotions that he is feeling.
What I've been learning with him, is that I need to anticipate when he getting towards an emotional whirlwind. It's about deescalating the situation, before it happens. Because once there, even the threat of punishment isn't a deterrent. It's been long and arduous. It's an inflexible thinking pattern.

He will still suffer consequences, but sometimes those consequences come after the melting down.

More prone, when he isn't getting his way, when tired, when hungry. When he feels that life has tossed him a big ball of unfairness. I've learned trying to create an environment where he feels he has a say, in compromising behavior. For instance, I'd like him to get off his gaming, and eat or share time with his brothers. It's up to me, to say, at such and such a time, I will need you off of this. I will then, set a kitchen timer. When the timer goes off, finish up, he has 5 minutes and will set the timer. It gives him a semblance of stability. That's one way, I counter having him do what's expected and then not having him as emotional. Rewards, such as a weekly allowance, even, aren't conducive to his emotions. I reward with each task expected. Otherwise, an entitled thought sets in.

I am not sure, if some of what I do, helps your situation. Remember, safety first. He's been prone to violent lashing out, as well. Creating a safe environment for myself, my other children and property matters.

Also, another thing, I've set up, is a consequence list, on my fridge. I add with each undesirable behavior. I list the behavior and the consequence, and it is posted on my fridge. There is another list, of the date and punishment duration. Keeps me consistent with following through with what I guaranteed would happen.