How does this build for Laevatein look?

You could do that, but I think you should go harder on her speed with brazen atk/spd (or wait for brazen spd/def to come out) or even darting blow because she's very likely busting anything and everything she doubles if you buff her well.

Also, if you agree with my idea that buffs enable her to hit very hard as is, maybe use noontime or even aether for some sustain, or go for galeforce and use heavy blade