Asking means it's an option. Stop asking for what's required, and only ask for what's a favor.
Consequences, but they have to be more immediate. And the consequences are their CHOICE to make sure it's phrased that way. "Okay, so you are CHOOSING to lose computer time."
Now, for getting ready for school, I simply told my son that he could go when he was ready. I said, "I understand you want to play and not get to school on time. So we will go when you are ready. Now, remember that the school rule is that you have to check in at the office instead of going right to your class. So when we get there, I will sign you in and you can go talk to Mrs. Principal and just explain to her why you didn't think it was important for you to be in school. I know she'll be interested in hearing it right from your mouth." He got ready immediately because he didn't want that CHOICE.
Homework? They are little so it's not like their entire academic and professional future is riding on this. So, just once, let them go with no homework. Just say some version of what I said to my son: "Fine. Your choice. You can explain it to Ms. Teacher in your own words. I'll stay out of it. Whatever her consequences are for unfinished homework, that's what you're CHOOSING. If you don't get to do X, Y or Z or if you stay in for recess, just understand that it's your CHOICE."
So, I'd take a quiet moment when you're not in a rush, and sit them down. Tell them you are done with the yelling, and you're not going to do it anymore. While you won't budge on safety, they're going to learn pretty quickly that it's better not to argue about things.
That means you have to be willing to be inconvenienced in the short run. It means being late to work once or twice, or at least setting the clocks ahead 1 hour so they are up earlier than they think. You can easily take the TV remote or whatever other electronic stuff they waste time with. You can skip packing dessert or making lunches if they have a purchased lunch option, since "I didn't do those things because I didn't think you were seriously getting ready." No yelling, just facts. If they yell at you (and they will), don't play along. Walk away. "Sorry you're upset, but this was your choice." The idea is to make things as inconvenient and undesirable as possible for them, without the right. If they whine in the car, give a warning that you aren't listening to this, that they are distracting the driver. If they persist (and they will, because they aren't used to doing things the first time), pull the car over and get out. If you have childproof locks in the back seat, great. Let them sit there and scream at each other. You stand where they can see you, so they aren't afraid, but not close enough where you can hear the complaining. If it's raining, see if you can pull into a safe area like a donut shop - and get yourself a donut while they can see you, but nothing for them. I kept a book of crossword puzzles in my purse for waiting rooms, and it was great for keeping myself occupied while my son figured out that his tantrum wasn't on my list of things to deal with that day. They'll get pretty bored with themselves in short order. Of course, they'll be late to whatever they are heading to - and they will deal with the consequences. And you'll get a donut because you "deserve it for listening to whining." And if they were headed to the mall or the park or the play date, but they DECIDED (CHOSE) not to behave, then it becomes their CHOICE to have you turn around and go home.
I think delaying the consequences (like losing something on Saturday for what they did wrong on Monday just delays the reality for them. The lesson has to be that they will have a lot more fun doing things your way than persisting in doing things their way.
This stall-and-delay is a game to them, and the only way to deal with it is not to play, and to change the rules on them. You ARE the boss and you need to act like it. It's hard but you do have all the cards - you just have to play them, and make the kids realize what cards they have been dealt as well.
Think of it this way - it's great practice for exerting control before they are teens and this starts all over again.