Hmm I got experience from just drawing without even thumbnails, and it can be fun but also time consuming if you’re a bit perfeccionist, because you want a result X but had no planning going on for this. lol BUT it’s fun and challenging. If you want avoid to having problems, it’s best to be prepared

About the amount of script, it’s nice to have a rough draft on the whole story and then a more detailed story written down by chapter/acts. And to have the dialogue boxes well determinated. And before each page, making a thumbnail with your main poses to speed up the proccess. :]