>>377838505
basically.
there may be a way to make shapekeys binary without the inbetween, but if there is I dunno how to do it. so im not sure how effective that method would be.

the surface would have to be sitting super close to the face mesh so your shapekeys can just move it in slightly or out slightly.

all depends what you;re going for, etc. I'm mostly talking out of my ass here since i've never tried that myself