That's an unexpectedly detailed response, in a good way. I can always rely on you for valuable points of view!
Everything you've pointed out is pretty much the same areas I've been looking at. In terms of character and main plot, I have actually worked out a loose plot-line that can potentially span across 3-4 books (or "series/seasons" - because I love the idea of making an ongoing story) and already have several ideas for sub-plot, character arcs etc - although it's true what you said that a lot of those become more harmful and unnecessary (plus it overcomplicates things). I do like the idea of keeping things remaining a mystery though, it would work very well with the Enigma character (who is essentially mystery incorporated), but I am going to revise on what is needed and what can be dropped (or used later).
In terms of page scripting, I like to try and write out a full chapter (or issue) in combination with page layout drafts (with isn't exactly a norm, but it does help me work out a good arrangement of picture and text). I have the first chapter mostly worked out in terms of writing, my biggest problem is starting the story - that old cliche of "it's hard to begin but easy to keep going" - but I've recently brainstormed a very visually exciting beginning to the story (my theory being it should help capture the reader and make them want to see what happens next), usually all of these ideas are springboarded off my wife - who pretty much acts as my quality control guinea pig. If the story makes no sense, or the jokes don't exactly hit home with her, I tend to know that certain parts need tweaking.
I do remember you mentioning perfectionism a few times on your instagram posts, hopefully that doesn't cause as many problems for you either. Looking forward to seeing Zombies become a fully readable book, definitely interested in picking up a copy - if the story matches the quality of the artwork (which it will, I'm sure), it's going to turn out great - and the whole "colourful apocalypse" still stands out as a cool concept!