As for "wide vs tall" character development, I can sympathize with the idea of seeing fewer people in more detail, and I certainly tune out whenever people start talking about uninteresting characters like Yamame or Cirno, but isn't that what the databooks and such are for? Also side materials like the manga. They let you focus in on characters who may be unimportant but flesh out Gensokyo as a place where not everyone is important. That's something I care about.
I need to think about this a little. That "Gensokyo as a place where not everyone is important" seems like something that should be established but something about ZUN's characters makes this very awkward.
The early stage characters are in a strange place in my opinion. On one hand they do flesh out the world of Gensokyo by showing the sort of stuff that inhabits the world. Mystia, for example, is one of many night sparrows, and is treated pretty inconsequentially in IN. However, she also sells grilled lamprey and sings in a punk rock band. These things can't be typical of night sparrows, so she does stand out from the crowd. So on one hand shes one of these unimportant characters and on the other she is her own character. Becoming her own character is a good thing, but in doing so Gensokyo, and more specifically the Night Sparrows, louse their mundanity that readers are looking for.
My answer would have been something like, introducing multiple Night Sparrows, maybe even in stage 2 of IN, and then elevating one with things like being in the punk rock band. That would further smooth out the gradient from important to unimportant character.