@ericp! Have you ever tried adding gum Arabic to your gouache? That adds a nice shine...although, it can extend the drying time (in my experience).
Yes I am aware of the GA trick, thanks for mentioning that. I agree that the shine does make a difference.
I've also found that GA helps with smudging once it does dry. Somehow I couldn't avoid smudging the gouache when I erased lines until I started adding GA.
Another thing I forgot to mention about gouache (having tried it again this weekend about a long, dry spell) is that it does smudge as you write (not just during drying as you rightly point out), which is, frankly, quite annoying when you cross T's or X's or any simple flourish for that matter. (I don't know if GA would alleviate this.)
I was also a bit disappointed with the hairlines too (using a Nikko G). My memory doesn't serve me well because I can't recall which magic nib I used to work with gouache. Oh well I'll find it sooner or later.