Maintain custom normals when deforming mesh

Apologies if this isn’t the right section to post this. I’m trying to create a specific NPR style and my latest test has gotten pretty much where I want by using heavily edited normals. The problem is that once I deform the face, it starts to fall apart.

Face in rest position:


Face with test expression:

As you can see, areas that should have planar normals now have visible polygons. Is there any way to preserve the custom normals when the mesh is deformed?

Have you considered using autosmooth and mark sharp? This should persist through deformation and not require manually editing normal angles.
Reference:

Actually, I reached out to Ruki, who’s one of the best NPR artists, who suggested I have a duplicate mesh that doesn’t deform and then copy the normals from the duplicate with a data transfer modifier set to topology. In my tests, it does exactly what I wanted.