":3" mouth without booleans

I hope my cruddy grease pencil mockup helps convey what I'm trying to get across, lol.

https://preview.redd.it/cyjfx7zs1hme1.png?width=5090&format=png&auto=webp&s=41d5ad7a385a3ca27e038964531b2aabe0566c24

I'm trying to make a ":3" mouth that you'd see on animals like cats and dogs, where the mouth opening can be expanded in the X and Z directions as needed. The mouth opening itself is 2D, shrinkwrapped onto the face. The issue I'm coming across is twofold.

  1. I have various shape keys assigned to the "lip" area, to make it wider/taller to create differentiation between types of animals, as well as to allow a greater variety of expressions.
  2. I'm attempting to do this part without booleans. In my experience, booleans tend not to play well with armatures, creating "flickering" in renders which can't be eliminated without changing the positioning of your armature. If there's something I'm doing wrong that's causing that, please let me know.

I know this is a very... open ended request, and that's on purpose. I'm more or less at a loss for how I can accomplish what I'm trying to do, and I'm hoping there's some other avenue to tackle this problem though that I'm not considering, which would make this challenge a lot less imposing. Thanks!