Definitely one of the finest advice. Yea, I doing thumbnail too even on my sketchbook. I have friend struggle drawing on sketchbook because he like doing on big canvas. Not saying drawing big canvas is bad, it's just if they don't know step by step, they just overwhelmed themselves by too many things to do.
Sometimes. Most of my published art is based on sketches I've drawn in my sketchbook. But sometimes, I make a small, quick doodle to get the idea across to myself. I did that most recently with an upcoming drawing, and it helped a bit.
I never plan ahead. I make one single finale sketch and colour that shit in. Add some shading to it and it's done... I've never been good at planning things out. I'm better at improvising. I know a lot of people benefit from using thumbnails though. So definitely not a bad idea.