Current sinking capability of HV507PG
If I understand the datasheet of the HV507GP well then it can indeed only source or sink less than 1 mA. Note that there is also a resistor in series with the NMOS drain for the HV outputs. It is unclear what the value of this resistor is but for sure it does not help in your case.

This IC is meant for electrostatic applications and that means high voltage but very low currents.

I'm afraid you will have to look for a better suited driver IC.