Hello everyone! Greg, I understand your frustration as I had a somewhat similar project where I needed to simulate realistic interactions between soft and rigid bodies in a dynamic environment. It involved complex hair dynamics and collision detection, but in my case, the goal was to create a believable animation of vines growing and wrapping around various objects with different textures and materials. And I struggled to get the vines to interact realistically with the objects.

First thing that helped me was experimenting with TyFlow in 3ds Max. It offers a level of control and flexibility far beyond the standard particle systems. I adjusted the collision settings and experimented with different forces and it helped me to achieve a more natural interaction between objects.

Much like your approach, I found that working in smaller segments and incremental iterations was key. Perfecting the interaction in a controlled setting before applying successful settings to the larger scene is very convenient, and it also makes it easier to identify and correct problems without overwhelming the simulation with too many variables.

Another thing is getting the material properties right. I adjusted the density, friction, and stiffness and fine-tuned the natural interaction, and you can do the same with your teeth/toothbrush interaction. It took several tries to find the right balance though)

Greg, I hope sharing my experience has helped you at least a little for your situation! Do not hesitate to write back, I will be happy to discuss the details