My lines are much better than my colouring, but I still want to improve both. I suppose linework involves both being able to know what sort of lines you're trying to achieve - which for me is heavily feral and natural looking - and being able to achieve them. Sketching from photos helps every time with the first, and sketching lots helps with the other. Tutorials help too, of course. I was looking through  falvie's sketches the other day; I love how definite the shapes look - I want more structural detail on mine.

I had the idea a few days ago that I could usefully leave a smaller copy of the line art in the corner of the coloured doodles, because i, but I haven't remembered yet.

It's good to draw what you enjoy, because that's where the drive to practice comes from, but it's good to challenge yourself with things you find hard because that's how you develop. There's nothing wrong with cartoons or anime, but they're both based in realism, so it'll make both better if you look at the original material to see how the stylisation has been achieved - it helps you see which bits of things aren't conveying the effect you want. Of course, you can learn the technical aspects like line weight and shading without anatomy.

Sycra on YouTube says something like how you want to get to the point where if you draw something particularly well, you should try drawing it again rather than marvelling at it, because you want to get to the point where drawings are almost throwaway things: when you were learning to write or type, you struggled over each letter. Now you think about structuring your sentences and paragraphs, while you're keeping an eye on the whole document.