I imposed the "no tantrum to earn screentime" on Monday and to my surprise, we've had two tantrum free mornings.
I expected a behaviour bubble when Nathan would up his bad behaviour in order to pressure me into dropping my expectations. I was prepared for weeks of morning warfare.
Instead, he's been polite and courteous and has earned his screen time.
This makes me think that the tantrums weren't as ingrained as I thought (though he'd certainly been persistent when there were no consequences). Or maybe his self-control is better than I thought. Either way, I'm taking the win.