In my humble opinion, most of the lesson 1 principles should actually be done everyday as a warm up and kind of become a part of your life as an artist, you can check Peter Han for that too. About the boxes I feel the same way, it's important that we understand them right away because they're so basic, necessary and prevalent. So even after your 300 boxes, you're still going to keep drawing them, so if you already have the approval to move on, just move on and keep the basics in mind and always work on them to go even beyond to more complex structures. :)

I also took around a year off whenever I started draw a box a couple years ago because I was starting college and decided to start over when I returned a year later. I honestly ended up learning a little more than I did last time and was able to build on that prior knowledge from the year before, it may seem counterintuitive, but I would definitely recommend just starting over from lesson 1 and working your way back through it, it will be worth it later on.