Power struggle 4yr old

The 4 yr old want to negotiate and power struggle over everything they are asked to do nearly, “I want to do it myself, she says.” She very much wants to be in charge. Food is an issue. She won’t even taste the food she is served, even if it is something she used to like, or is something new. At bedtime she always asks for one more minute. Or three more minutes. Every time, I say the three minutes is up, she wants another minute. If I say, “No”, we might have a tantrum. We frequently get tantrums on my watch. Mom will take punitive action if there is a tantrum starting. So, she gets less power struggle and tantrums, but she still gets some. If I get a tantrum I go with the “let’s get calm” approach. Then we can talk about it. How can I improve the child’s behavior in general? The two year old is following suit with the four year old, so if I can correct the behavior now, that should pay off for the next child.