FWIW - in my experience, if their demands upset you, it’s not going to go well (you’ll get more demands and feel more exhausted); if you laugh at the ridiculousness of the situation, remind them to ask nicely, and still help them with what they need/want, you’ll be much less stressed, you’ll be teaching them that being unpleasant doesn’t bother you and doesn’t help but does have the natural consequences of slowing down what they want, and you’ll be teaching them a better way of being; at least with my own kids, they picked up on it pretty quickly, and they started being much more fun to hang out with.
True, and oddly given my comment, I adore them and love being their dad. But they destroyed my old life so I've had to build a new one around them. The pandemic hasn't helped.
(Good luck!)