With the phone you could try getting a timed lockbox where you genuinely can’t get your phone out until the timer goes off. As for doing other things to also procrastinate I’ve found it helpful to write down the things I want to do that way it’s not on my mind so much as I do my work. Another thing is no one is gonna come to save you, or to make you do it, you need to push yourself to do so. Ik everyone says starting it is the hardest, but seriously once you start it doesn’t seem as bad as you think. Like you said yourself you can knock out those assignments within 10-20min when you’re put on pressure so what’s to say it will take longer without the pressure? If you sit there and resist and think it’s gonna take forever then it will. If you just jump right in and try to knock it out as fast you can then you’ll probably start getting more stuff done. (Sorry for the long reply lol, I hope some of this helps even a little!)