Hey there waldy, your comments and discoveries are pretty spot on and I'll address all of those in this critique! =) Let's get to it.

So overall you have made some good progress in terms of your sense of space and the solidity of your boxes. One thing I am noticing is that sometimes you have a tendency to rush and play fast and loose with your lines, both in hatching and in the lines that make up the boxes themselves. Remember part of the reason we use pen is to respect line economy so make sure you're ghosting and planning each and every line.

In terms of your converging lines, you did have some improvement and as you say there is still some issues with the back lines which is common for students. Uncomfortable has made this infographic to further expand on how to approach parallel lines in perspective. The first thing we must think about is how parallell ines are a family all tied together by the vanishing point. You cannot change anything in this family without it having an impact on everything else. Often times students will start a box with a plane and make sure it's all nice and well then move on to the next with no regard to the lines they have already laid down. This often results in multiple points of convergence instead of one. What you need to do is learn to "step back" and see all the lines both drawn and not drawn yet. Admittedly, this is very difficult at first but as you practice doing this purposefully it will start to become more intuitive and things will start to fall into place more.

You did a good job throughout your challenge playing with different sizes, orientations, and rates of convergences for your boxes and the only thing I would have liked to see is some larger ones. With that in mind, try to keep up your box practice in warm ups as the next few boxes are very light on them, and when you do boxes try to do some larger ones (like 1 or 2 taking up the full page) to get acclimated to the unique types of spatial problems boxes at that scale have. It will come in handy for lessons 6 and 7.

That being said, your box challenge is now complete. Congratulations.