Ok, so you know she can regulate her emotions as she does it at school and GPs, and she used to at home. So you are right that’s good. I wouldn’t worry about the voice in her head - could be school are starting to explain to the kids how to recognise emotions and inner monologue.
So now she shouts at you when she is tired and can’t seem to regulate her emotions any better. That is normal. Home and parents are her “safe space” so if she acts out anywhere it will be at home. She relies on you to keep firm boundaries for that sense of security and a phase of tantrums won’t hurt her or you.
First I’d consider - is she genuinely overtired and/or overstimulated? Does she do after school club and extra curriculars too? Is she in a growth spurt? Happy at school? Wobbly teeth hurting? Poor sleep due to nightmares? How much time does she have that is empty so she can simply play and unravel a busy day?
Does she tantrum less when she’s relaxed and definitely not tired?
I would consider what kind of things she is tantrumming about.
with my ds If i say, “please tidy up your toys, it will be bathtime in five minutes” he will probably kick off. If I say - “I’m going upstairs to run your bath soon. Once you have tidied your toys, will you come upstairs and choose the bubble bath you’d like tonight?” He’s immediately thinking about how nice his bath will be. He won’t tidy his toys but he definitely will bring himself upstairs and engage in bathtime. So I have figured out how to push the right buttons there.
Meantime his daily phonics homework is a living nightmare - he tantrums about doing it every day. So I just calmly remind him it isn’t optional, and i ride out the storm. Perhaps one day he’ll just cave and come quietly but i doubt it.
I only use a time-out if I am getting so angry I can’t trust myself to stay in control of my own temper. Sometimes ds takes himself off to cool down. I don’t usually find a time out works - it just makes the howling and seething sense of injustice worse in my experience.