Marlin 1.0 works: Marlin 2.0 destroys Z-axis motion
I believe (from reference) the default steps per mm for the FLSUN 3D Cube are:
                                          X,   Y,    Z, E0 
#define DEFAULT_AXIS_STEPS_PER_UNIT   { 100, 100,  400, 150 }

Default marlin 2.0.x are:
                                          X,   Y,    Z, E0 
#define DEFAULT_AXIS_STEPS_PER_UNIT   {  80,  80, 4000, 500 }

Such a setting would explain the excessive speeds; while the printer only needs 400 steps for advancing a single millimeter, the stepper receives 4000, this implies a tenfold, hence larger displacement and higher speeds.