I was actually going to ask you how concrete your scripts usually are, but this seems to have answered that.
If it’s something quick and self-contained like MGDMT, I plot out the panels and text at the same time, so my “scripts” just look like this;
If it’s a short story or something like that, I draw thumbs with story beat notes beside them to kind of remind me of what’s going on in different panels or dialogue or whatever.
If it’s long-form stuff I brainstorm really loosely, write down plot points on things like post-it notes I can scoot around as need be, write chapters in point form if I still need to better solidify how I want them to go, and then do really detailed screenplay-style scripts that I use for making my thumbnails.
(That screenplay software is Celtx, by the way, it’s lovely for scripting)
I never feel like the first pass at dialogue sounds natural enough, because at the time you write it down you’ve just been stewing over it in your head. Even if it’s the first thing you think of, you’ve had more time to consider about it than a person would reasonably have in conversation. People need to breathe and people need to think, and usually the pacing on either of those things doesn’t seem right unless you’ve had a chance to write it down, clear your head, and then read it again at normal speed. Once I have my “final” draft of a script, I’ll start reading it from the beginning. Any time I stop myself to correct something or revise a line, I’ll start from the beginning again, just to make sure I know what the story flows like in “real time”