First, those are some really harsh punishments. Punishment rarely works, and now that you've seen for yourself that it doesn't with your kids, please stop.
I don't know if there's a good English term, but you've reached what pedagogues in Sweden call the method ceiling, we're your current methods are no longer working. At this point we usually escalate our methods, like you've now escalated punishments in desperation to rather extreme levels. For those who employ corporal punishment, this is where they feel compelled to escalate the violence because they don't achieve obedience. When you have one tool and it's no longer working, it's easy to default to just applying it with greater force. This is silly. You never want to do more of what's not working. Do less of what isn't working. You need to find yourself a new set of tools.
Second, children don't generally learn from their mistakes. Learning from mistakes is how we fine tune our skills, but children are more often in the process of acquiring skills. We learn from the instances where our expectations are not met; where we are surprised by an outcome. As adults, succeeding is the norm for us in many tasks, so mistakes are notable and informative. We can compare them to what normally works to see what went wrong. To a child, not succeeding is often the norm, and another failure doesn't carry meaning to them. If the infant trying to take their first steps would learn from their mistakes they'd learn that they couldn't walk, and stop trying. But they're in skill acquisition mode. They don't expect to succeed. They keep on practicing. Children learn from their successes. If we want to help them learn, we must first help them succeed.
The switch to when they start learning from their mistakes varies from child to child and from task to task. But learning to plan ahead and avoid distractions and perform unmotivational tasks to a fixed schedule is hard. And the punishments are so harsh they'll overshadow anything they might have learned. If you want them to learn from their mistakes (which, again, they may not be ready to) you want the failure to be the bad outcome. "We procrastinated so now we missed the deadline". I don't think they'll even notice the missed deadline, because they'll only be able to think of the punishment. If anything, they'll learn that they are growing up in an environment where there are draconian punishments associated with failure, which is out of their control, and develop coping strategies for that. I really think you're setting them up for failure.
Third, rewards are something you typically might employ when trying to get over a threshold. Something that, once acquired, the child won't regress on. Exceptional things. If rewards are a part of daily life, it is more like any salaried job. With that comes inevitably that, a) the child will make an economical judgment on whether or not the goal is worth achieving, rather than being motivated to the goal in itself; b) there will be an inflation (just like you might expect an occasional salary increase for continuing to do the same job); and c) that you'll have to keep this up: we often have an idea that once the desired behaviour is reached, we can phase out the reward, but again, imagine if your boss tried that on you.
And for all the effort I could put down arguing, my point is something you've already noticed: your tools aren't working for you. You, the adult, can learn from your mistakes, and stop relying on rewards and punishments if it isn't working.
Instead, you want to foster cooperation. Assume they're doing their best. If they can't focus on a boring task, or abstain from a distraction, then work on helping them, rather than punishing them for their shortcomings.
If they fail, assume you've set the bar too high. Claim ownership of the problem. If you think they are the problem, then you've rendered yourself powerless. You can only hope they'll change. If instead you choose to see that you have a role in setting them up for success, then you can work towards that as much as you want.
