MANY yrs ago (maybe 12?) I attended a Highlights weekend with Matt Phelan talking about Graphic Novels. If memory serves (these days it usually doesn't) his submission to Candlewick was in a movie script format, because he went to Uni to study, hopefully write, movie scripts.
Since many GN's might only have text moving the story along in less then 1/2 the book, a script can be used to describe panel by panel, pg by pg, what's happening in the story. Google Matt and there are a number of interviews (like for "Snow White", at the Miami Book Fair) where you'll get glimpses of inspiration on setting up your Manu"script"

For a GN all thumbnail sketches need to be completed IMO because they are in essence, your text and you always complete any other manuscript before submission. BUT nothing needs to, or should be, final art.
For your portfolio, SCBWI has on-line events from time to time that pertain to them. I just saw this one, and reg closes tomorrow (Friday) coincidently. It's $50 for members but does sound very good, because of the Art Director doing it. I'm sure the 15 private portfolio reviews are filled up by now but doesn't hurt to find out. Either way, you can be part of the on-line course.
https://nebraska.scbwi.org/events/scbwi-ne-illustrator-intensive-2022/Do check with your regional IC to find out what other events might be happening for Illustrators, specifically for GN's and Portfolio. (you can also spend time going region by region to see what up coming events they might be hosting) Good luck!