Your 8yo sounds exactly like my daughter. She was a very pleasant kid, never much for tantrums or digging in her heels and saying "no!", so so we never thought of her as being particularly strong-willed. We just put any failure to comply with instructions down to her busy, extremely active nature, and would because she was so verbal and curious we found ourselves trying to explain and discuss everything to death.
But at a certain point our preschool director gently pointed out that she was walking all over us in a kind of passive-aggressive way, and had kind of unconsciously figured out exactly how to manipulate us. He recommended this book, and no lie, it changed our lives:
https://amzn.to/33fbDcn.
It won't solve your problems in the short-term, of course, but if you start implementing some new strategies it might help you feel like you're doing something proactive. It won't work if you don't stick to it, but the stuff he recommends should help out with all the kids.
It's nothing groundbreaking, mostly boils down to setting out clear expectations and consequences and following through every time, but it helps to understand why certain kids act and react the way they do.
But there's also a section on temperament, and how to adjust to accommodate both the child's and the parent's temperaments. That might actually help a bit in the short term, if it gives you a better understanding of your own reactions to their behavior.
Hang in there!