I wonder if the smaller wire spheres in shaker bottles would work well. I'm curious if small enough ones exist that could fit inside. Also there are the super small bearings used in nail polish bottles that would accomplish the same thing as a marble and they are meant for mixing thicker liquids in glass.

Edit: https://www.amazon.com/Stainless-Steel-Mixing-Balls-Polish/dp/B006PPJUEQ