I discovered that Keynote works even better for storyboarding than I thought, if you have an iPad and Apple Pencil.
I should clarify that I am not only illustrating but writing a children's book so I needed something in which I could see both text and a sketched thumbnail in order to get a sense of the narrative flow and rhythm of the story. As I mentioned above, I ended up using Keynote (Mac's equivalent of powerpoint.) I customized the slide dimensions to be similar to my page dimensions and then put the draft text on each slide. I used the slide table to get a feel for how the text was working and added/deleted/or rearranged the "pages" (slides) to make the text work.
I also discovered today, however, that Keynote on the iPad allows you to draw directly on the slides so I am able to do the rough thumbnails right on each slide which is a very nice feature. (The drawing tools are very basic but my initial thumbnails are really rough so the tools are fine.)
Although I can't see specifically formatted double-spreads, I can see the thumbnails side by side to get a general idea of how it will look. (If I had a lot of double spreads, I would probably custom size the slides to the width of a double spread and draw the thumbnails for each side of the double spread instead of each page.)
Once I am happy with the text and initial sketch ideas, I will do more elaborate thumbnails for each page in Procreate and then can erase my really rough sketches and paste the final thumbnails into the slides.
All of this is probably an unnecessary method if you already have set text but it is really helpful to me as I am working on the narrative as well as the illustrations.