synch_throttle.py

import time
import threading

import gremlin
from vjoy.vjoy import AxisName

# Implement using recurring timer
def slider_update_timer():
    vjoy = gremlin.input_devices.VJoyProxy()
    joy = gremlin.input_devices.JoystickProxy()

    vjoy[1].axis[AxisName.SL0].value = joy[3].axis(4)
    threading.Timer(1, slider_update_timer).start()
#slider_update_timer()

# Implement via sleeping thread
def slider_update_thread():
    vjoy = gremlin.input_devices.VJoyProxy()
    joy = gremlin.input_devices.JoystickProxy()

    while True:
        vjoy[1].axis[AxisName.SL0].value = joy[3].axis(4)
        time.sleep(0.5)
t = threading.Thread(target=slider_update_thread).start()

I've narrowed it down to the code not correctly accessing the current value of the physical slider.
I can access my second joysticks slider.
I can set a macro to occur between value on the slider in question.
The code just always reads it as -1 (confirmed by display_error function).