Wish author wasn't such a chicken if it was intended for body parts, show the demo in the README, I think it's still legal isn't it?
One question: how does it hold up when the framerate changes? Spring jiggle like this usually either blows up or feels different when the timestep moves around. Are you using a fixed timestep, or just relying on the damping to keep it stable?
That is one place where an analytical solution is a benefit, even if it is a bit less realistic. You just have a position(t) parametric function you can evaluate when rendering sporadically.
Why would the physics timestep move around? Typically you keep that fixed and separate, especially not locked to the display framerate, physics get really wonky then regardless of what you do.
Makes me wonder: Does the adult entertainment industry have its own variant of the military's DARPA, or is it truly decentralized in its innovation and standards work? Haha