I mean, there’s no such thing as a perfect insulator

I’m well aware that nothing is perfect. i was talking about definitions :p

It definitely resists the movement of free elections through… But think a capacitor let’s ions flow, grease is a sort of fluid…

the two terminals of a capacitor are insulated from eachother but they can pass alternating current via capacitive coupling. I’m not sure what you mean about flowing ions in grease. do you mean electrolyte? like in a battery?

So I’m thinking it must be a material that let’s atoms move around to some degree, but resists the transfer of electrons

you’re describing electrical resistance :p