Instancing a greasepencil frame? (or alternate workflow for lip-syncing)

I’m working on a 2d animation for a song and just finished doing the lip-syncing. Using a few frames with my basic mouth shapes, I went through the song and duplicated (shift+D) the appropriate frame to match the lyrics.
My problem is that alt+D (for making an instance) doesn’t work in the greasepencil dopesheet. That means that each frame is independent, and I can’t adjust a mouth shape in one place and have that change reflected on the whole video.

I thought that next time I could put each mouthshape in a different GP object. That would solve the “instancing” part of the problem, but would become a nightmare for animating (would need 3 keyframes to toggle visibility for each single phoneme!).

Does anyone know a GP lip-syncing workflow that would solve that problem?

Alright, found out how to do this, should have searched on youtube first:

Basically, you just add keyframes to a time offset modifier set to “fixed” that just affects the mouth layer.