Congratulations on finishing the 250 box challenge!!

I've just done my own boxes, and can tell it was quite a marathon : )

I see you correctly got the basics of this exercise. Your checking lines always extend away from the viewer, and every box has one hatched side, which is really helpful in understanding the orientation of the box.

Also, the lines of your boxes always converge, which is absolutely correct. And here I noticed that you prefer the dramatic foreshortening. It could be a reason why sometimes your boxes go wrong. For me, it was more difficult to calculate correct angles using dramatic foreshortening. I'd suggest you in your future exercises concentrate more on shallow foreshortening. It could help to understand the lines' interrelations better.

The second tip that could help you build more correct boxes in the future and better understand 3D space is to change the order of your box modeling, namely to draw the inner corner first, as shown in this instruction

And in the end, I should also mention that your lines are pretty wobbly. That's understandable for the beginner and absolutely fine at this stage. The best way to improve them is to use the ghosting method regularly and watch yourself to keep drawing by the shoulder.

Overall you did a great job!

Go ahead and keep practicing!