For the first 50, your lines are clean overall. However, I did see some wobbling throughout the set. Your convergences also show good accuracy, though I did see a few lines veer off in the wrong direction, like in box 6, box 29, and box 48.

For the next 50, your lines show improvement. While some wobbling is still present, most instances are less dramatic overall. Your convergences also show improvement in accuracy, with fewer errant lines. However, I did see some lines pair off and converge towards two vanishing points, like box 53's green lines, box 75's red lines, and box 91's green lines.

For the last 150, your lines continue to improve, with wobbling notably reduced throughout the set, especially towards the end. This demonstrates very good technique and consistency. Your convergences also continue to improve, though there are still instances of lines pairing off, like in box 107's green lines, box 180's blue lines, and box 228's pink lines. In these boxes, the inner lines (green and orange in this diagram) diverge instead of converging at the vanishing point. Keeping in mind the angles of neighboring lines can help when planning the box's construction.

As an aside, some of your boxes are missing some line extensions: box 21, box 54, and box 217. It's not a major issue since it didn't happen often, but I want to point it out for future reference.

As for your concerns, it may not feel like you're progressing in the moment, but as I said in the critique, I can see that you are. Also, the goal of this challenge is not to draw perfect boxes. It's to practice the line-drawing technique and hone your ability to envision correct perspective. You will improve further over time as you practice.

Keep in mind, also, not to worry too much about those back corners right now. As the last point drawn, that's where small mistakes will build up. Focus on your technique and getting your lines to converge as well as possible. For the lines that pair off, if it's because the back corner is off but the other three lines converge well, don't worry about it. If they pair off because both inner lines are off, keep that in mind for the next box.

All said, you did very well. I'm going to mark this challenge as complete.