Here’s a handy tip, drop some quick lettering over your thumbnails to clarify storytelling BEFORE you draw the pages.
You’ll have to put the text on the page anyway, might as well do it as early as possible, and that will allow you to kick up the quality of the artwork because early lettering shows you all sorts of things that can be impossible to see otherwise.
I find myself changing positions of the characters, tweaking facial expressions and body language, adding and/or removing dialogue, etc. All the problems are solved in the thumbnail stage, which is why we thumbnail in the first place.
Once the final artwork is done, I swap out the thumbnail page for the finished art and make a few tweaks, then I’m DONE.