Super.
I think line quality is like a voice in singing or style in playing a guitar. It's all about character and tone. And of course, it is interesting to match a line quality with a subject, and when coupled with the stylization of the characters you create a distinct voice.
So many drawings exist in our culture that picking one will often create associations with art that existed before. I think one style that comes to mind is the famous Zap Comix, especially the stuff drawn by R. Crumb out of San Francisco in the 1960s. It was identifiable with the hippie counter culture. It's as identifiable to me as listening to a Janis Joplin recording and those artists who try to emulate her style. Similar to Jimi Hendrix's guitar style that so many artists have taken on. And then the new artists who emulate their style then go on to new things in that direction or they keep expanding within the range the original artist did. It's like so many other areas when there's an appetite for a look, somebody will generally go there.
Yes. I agree, the animator's line (Popeye above) is an identifiable look, and it came about from having to paint within those lines for film where they needed to match countless drawings one to the next without it popping on film. It's no longer necessary to have a closed line anymore because the ink and paint process has evolved with computers, but it still is used a lot because it's practical. . . and it looks good in that it describes the form but generally doesn't draw attention away from the cartoon character. So that page with the rules shows how to make Popeye look like the way the professionals do it (and some artists aspired to be animators and had to learn to draw that way). But it certainly is not the only way. But it gives a recognizable professional animation voice that has use in other contexts.
It's delightful to experiment and catalog ways of drawing as you're doing. I'm very impressed. It's quite a gift for people aspiring to drawing to share your process.
Bravo!
"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream