Always start your illustrations in the middle (to kind of warm up) and save the cover and opening spreads for the end (when you’re in the zone and it’s flowing) – because those are the first ones people will read!
I’ve done it both ways now: The Artifacts was illustrated as I felt like it, but I’d been putting off doing the title page because that required design skills which are a little different from illustrative inner pages. So for Midnight Feast I did the splash page, title page, menu page, navigation pages, about page, options page… and I still hadn’t started illustrating the story. This took several months and was a dispiriting thing to do! Next time I’ll be starting in the middle.