TBH I wouldn't use a sketchbook. My view is that the drawings, once completed and lessons passed, will be recycled.

I am doing all mine on copier paper. I suspect that sketchbook paper is possibly more absorbent compared to copier paper and that would account for the ink not flowing quite as well.

Get a stack of printer paper and a cheap clipboard to hold the paper you are drawing on. Only put a single sheet of paper at a time on the clipboard to get the best results. If you insist on using a sketchbook then insert a clipboard (or a similar hard+flat surface) under the page you are drawing on.

As for making the ink flow well, make sure you are holding the pen as close to vertical as you can. Switch your grip on the pen if required. You do not need to use your writing grip for doing DrawABox. You will be drawing from the shoulder so having control of the pen with your fingers is not required, you just need to hold it steady+stable.

From my experience, staedtler isn't very flowy. So some fade is just natural for these pens. Even on 80gsm standard print paper. But if this is out of concern - look for paper of density of about ~80-100gsm. But I really would advice you to get some simple print A4 and a clipboard.

This could be a result of pen quality or the type of paper you are using. Canson sketch books usually have labels that indicate what drawing instrument works best with the paper found in the sketch book. I learned this the hard way when I was using the wrong paper with my pens. The paper was absorbing all the ink causing my pens and markers to dry out too fast.

Laser-Inkjet 80g CLAIREFONTAINE paper works fine with me. The paper drink ink not to much and enough to avoid stain on the paper.