Dear Shannon,
My son is 21 months old too, and we're dealing with the exact same issues! (Well, not scratching and hair pulling, but a sudden onslaught of yelling, tantrums, and hitting.)
I don't have all the answers, and I'm sort of figuring it out as I go along, but I will share a couple of things that seem to be working for us, at least most of the time.
The thing that works best for us is the word "mad." When my son hits or yells, I say, "Noah, are you mad?" He'll then usually say "maaaa" and calm right down; it's like he's found something that's just as powerful as yelling and hitting. Every time he says "mad," I give him a lot of empathy and try to voice what's frustrating "oh, I understand, you're mad because your banana is all gone. It's okay to be mad, but we don't hit." Sometimes he'll even say "mad" all by himself, without hitting and without my suggesting it, and I consider that a successfully avoided tantrum.
The other thing, which I've just started trying, is choices. Last night, I said, "Noah, you have a choice. If you want to be with grownups, you can sit at the table with us. If you want to run around and play, you can run around and play by yourself." I don't know if that'll work long-term, but it did last night.
And when all else fails, I do what you do -- quiet time in his room with soothing music. I'm so glad you don't do traditional time-outs and that you don't construct quiet time as a punishment; 21 months is still mostly a baby, after all.
If you don't mind, please contact me if you find any other solutions you really like. I think we're in the exact same boat!
All the best,
Mira