For Z you would likely need to adjust the steps per mm, speed, and accel for the Z axis in the controller (Edit > Machine Settings).
For air assist, that one I’m not sure of. I know that it does work on both firmware, but the output pin might be different. C3D should be able to tell you that.
You might only have a 2 axis build of the GRBL firmware. Z and A axis might need a different build, but C3D should have the 4-axis version on their website for download. If the steps per mm are really small, the motor would move, but the Z table would barely move at all. You might also need to set more current for the Z axis driver.
When running one of my grbl-lpc firmware builds with external drivers, you must disconnect the stepper driver enable line for the driver to work. External drivers require a active high enable to work.
M8 air assist is Pin output P2.6 on the mini. M7 is not enabled in any of the grbl firmware builds.
To make the external driver work with grbl-lpc, just disconnect the ENA+ wire on the driver side. Wrap some electrical tape on the end to keep the wire from shorting something else. This should fix your Z table issue.
If the rotor is locked then the driver is “enabled”.
This is likely a wiring issue with the step and direction. Please verify everything still works using smoothie firmware.
Edit
I just checked the grbl-lpc firmware with open drain Z using a external TB6600 stepper driver. Wired using the Cohesion3D document. The enable line was left unconnected. It works fine here. Make sure you download the correct firmware version from my link above.
You may need to invert the Z step pulse to work with your driver.
If you are still having issues, I suggest you keep using smoothie since I can’t give much hardware support. The firmware works on my end.