You're pretty on point with why its looking like a cube I would say, if you had maintained that same level of rotation per box it would be more spherical but of course take up much more space on the page. Personally, my method of avoiding that is after drawing my first center box, I take a look at the edge squares then ghost out about how large that last box I draw is going to be, visualizing an estimate of its rotation. Then I do the same for the next box until Im ready to start my first actual rotations, if that makes any sense, and I understand its a method that might not work for everyone. But trying to visualize what something will look like on the page before touching it, I think, is a great way to get better with drawing something that has to have a relation to an object near it. Keeping track of a busy page does get tough. typically in my exercises, when I have a set of steps to follow for a box on the page, I try to just focus on that box until its done. Granted you dont want to lose the big picture of your page as a whole, as comparing to other parts can help you better guess vanishing points, but I will often keep the lesson page open on my computer and sort of make a mental check list of the steps before moving on. But It didnt look like you were having a major issue with that, just a slip up here and there, so keep in mind that being able to keep track of it all is a skill that will get better with time simply through regular practice.