When you can hear those switches clicking, that means the Actuator Screws are adjusted well enough to enable the switches to Close and Open.

You found that the voltage decreases, from 12 volts to less than 9 volts, when the switches are closed.

That much of a voltage drop tells me that there is high resistance between Common and Normally Open, when the switch is held closed.

I suggest replacing all 4 of the Directional Switches, plus the Brake Switch and the Door Switch.
If you buy these parts from Crown Lift Trucks, my guess is that each switch cost approximately $12.

The next potential problem is that you might have a problem that you can't diagnose, until new switches are installed.
Replacing switches is relatively cheap and effective.
If you can do this yourself.
Another service call means more money, and the Tech that shows up might not truly understand how to deal with this problem.
Take lots of photos of the parts we have been discussing, before you begin to disassemble anything.