Agreed on what @snakker said. Many of us started with very hairy lines (some of us still do so for sketching), but with practice, self study and time, you will naturally gain more confidence in doing those long stretches of lines. Especially since in digital you can just undo and retry as many times as necessary until you get that "perfect line".

Stabilization does help though, you can adjust is as needed until you feel comfortable with it.

The black background is an interesting choice, but it's definitely easier on the eyes than pure white. (I never use a pure white canvas as it hurts my eyes, too.) Gotta say, I don't really see any difference between the figures you drew from imagination and those drawn from reference - the poses look realistic and plausible.