Congratulations on finishing the 250 Box Challange.

I want to start by saying I love how confident and clean your lines are. The line weight is well done as well. You might want to change the variety of the boxes a little bit and experiment with different-sized boxes and different amounts of foreshortening as it might increase your understanding of perspective. Still, your overall understanding of perspective seems very good.

The correction lines are extended correctly. There are some divergences in the inner corners of some boxes. This order of drawing boxes, might help you draw more accurately, but the accuracy gets better towards the end.

As for your question, I don't see much difference in your line quality between the first pages and the last ones. What I realized is that, towards the end, it seems like you fixed some of your lines to be more accurate. Fixing the lines creates messier and less confident linework, so that might be the reason you feel like your lines changed towards the end. I think it's good that you spent time understanding the perspective better and tried to fix the specific problems you encountered. That's how you improve.

And for the linework, If you believe that thinking too much about accuracy holds you back, I would recommend you think less about the correctness of your lines or your boxes. Once you planned out the line and put your pen on paper, don't think about the accuracy anymore since you already made the plan beforehand and ghosted your line enough for your muscles to remember the correct move. When I feel like my lines are getting less accurate and less confident, I would stop drawing for a bit, and try not to think about the accuracy, and only rely on my muscle memory I build from ghosting the line.

I am not sure if this answers your question, but if you feel like I missed the point feel free to ask again.

I would recommend you to start more advanced box exercises introduced here and include drawing boxes in your warm-ups. Good luck. :)