I'm an amateur at drawing and even then, I use a different style, so I'm afraid I have no advice on how to manage beyond "layers", which is completely unnecessary advice since you already know how to use them.
Creating an "expression pack" seems like a great idea! I'm not 100% sure how you intend to go about it, but a nice range of expression is definitely something you don't find often in free character packs, which kind of gives you the advantage if you can offer them. "Sole seller" and all of that.
Possible expressions could include the following. I made a large list to be inclusive, but the first 3 are what I feel are important.
Angry
Surprised/Shocked
Scared/Nervous/Sweating
Tired/Exasperated
Shy/Nervous/Ashamed
I was thinking of using Sumi, Sora, Aiko and Kousei for the project, as protagonist and deuteragonist (so that the user can have the choice to pick the MC's gender), so extra expressions for them would be a godsent.
And don't worry! I'm always willing to answer some questions (as long as you don't ask where the chocolate went), so I don't mind at all! I love talking with people about stuff I'm passionate about.