Good work completing this challenge!

Your lines are all very confidently drawn. Even your earliest boxes have this on full display. While hatching lines on one side of the box was an optional part of the challenge, it was a requirement for you to apply line weight to your boxes' outer edges. This is something I noticed was particularly lacking in your submission.

The foreshortening on your boxes is present, but even the later boxes seem to have obvious problems with certain faces' trajectories. While confident line strokes are the bedrock for building effective fundamentals (and of which is something you've doubtlessly got in spades), don't be afraid to take as much time as you need to measure your lines before you draw them. Make sure you're witnessing the vanishing points in your mind, make and correct marks on the page to guide your lines, and then lay the lines down.

Remember when drawing in three-point-perspective, you're not going to have any sets of parallel lines. Some of your last boxes still had problems where pairs of extended lines appear to be nearly or completely parallel, notably pairs of lines on the same side of a box that are across from the longer line.

If you're having trouble figuring out how to properly envision your vanishing points when drawing boxes, ScyllaStew uploaded a video that may prove useful. After drawing your initial "Y" with each box, make sure each of your lines afterward all line up with that side's vanishing point, even the lines that are being drawn on the invisible back corner. This video really helped me change my own mindset for this challenge, so perhaps it may be helpful for you in some way too.

Finally, something I noticed in your submission is that you drew more than six boxes per page. If you have too many boxes on a single page, you're limited in the length of the lines you put down on the page. This means you can only draw one or two large boxes per page at most, meaning you have less practice with drawing larger boxes in perspective. Beyond even just boxes alone, larger objects drawn in perspective tend to have vanishing points that are farther away than smaller objects, so it would behoove you to try out drawing some larger boxes for this challenge.