the tank seems to be unable to actually move from a standstill if you only use the forward/backwards movement keys, needing to twist about a bit just to get it moving

That's a common issue due to the high mass of it. I want it to drive slow so that was my way of solving it. Maybe one day I'll fix it somehow.

my thought is that have the mass set so that it is just light enough to allow it to move on its own.
the other way is possibly giving the fuel it uses increased acceleration modifier, as that seems to improve the torque of the engine