In regards to putting things on her plate she doesn’t like, that’s how I parent, I always put a little bit of everything on their plates… I’m not going to stop doing that.
This worked as long as your dd felt safe. She doesn’t feel safe right now and you admit that yourself:
she’s gone from a very happy, well-behaved little girl to angry, sulky and short-tempered within a matter of months
She has had a lot of change and witnessed nasty abuse. This means she needs extra loving care, not a mum, who is doing what she’s always done.
She’s already struggling and have taken something away she was really looking forward to you. It would have been far better to have discussed the hitting and attitude with her. And looked at your behaviour as well. Not adapting to her suffering is punishing her for being abused.
You said yourself your parents were very harsh. Not adapting your parenting when your child is crying out is also very harsh. Do you want your kids to grow up believing their mum is harsh?
I’ve always said I’ll cook one meal and one meal only. I just hate the idea of sending her to bed hungry.
Right now you need to parent differently. I don’t think you should 100% have red lines right now. At the very least, the meal should be something familiar, which makes her feel safe.
My mother had the ungrateful child, you had it so much better than me attitude toward me. Except I didn’t have it better than her. She learned the concept of unconditional love, which I never had. Don’t be that mother comparing their childhood to yours and setting your benchmark as better than your mother so all fine.