Your line quality already is really good! I think it's just your mindset that you apply it :) Just try to keep up with your practice and you'll become more confident gradually. You could perhaps look into some books of Alphonso Dunn. Your exercises reminded me a bit of them. Keep it up!

Hi Vincent. If you feel like you are not getting progress on line quality. You just need to keep working, but also rest the hand with some automatic drawing. It is actually a way to improve line quality. If you don't feel like a noticeable progress, it means that you have entered in your slow peace zone of improvement. If you don't know, it is the second stage of any skill that you build through time. At the beginning you fly on progress and it starts to going down on speed. But actually you are still learning. You are mastering the thing, and you needs to take care of it may be more that before. So, practice smart and keep up the work. Probably you get some usatisfaction feeling at the start of the stage. But keep the practice, may be just focus on other thing instead line quality, but taking care of it at the same time. You get what I mean? In any case, don't get discouraged and practice as always

You're already well on your way! It takes a whole lot of time and practice to develop bold confident lines, that are also precise. And the 'only' way to get there is to draw bold lines as if you are already confident, but slowly, focusing and paying attention to what your body's doing so you can spot the source of mistakes and correct them. Simply put, make a mess, be bold, but don't stop thinking. Finding the right speed is something you gotta discover through trial and error though. Slow down as much as you can, to not be sloppy but not so much that you start overthinking your movements. It's a tricky balance. Also, looking at the pictures, it seems like you're drawing fairly small, possibly from the wrist as well. While it's good you're using a pen and doing some Drawabox.com stuff, make sure to also do the drawing from your shoulder and using bigger strokes part, draw much much bigger stuff, like 3-4 times as big. Just keep truckin' and enjoy the feeeel of the line, this skill can be an endless pursuit as you'll continue to raise the bar for both precision and boldness of strokes.

Really love all this practice. From what I see your line is really looking rather confident to me and practice like this will only make it better and better.