I have been posting about my ongoing problems getting brightness to work in High Sierra using AppleBacklightInjector.kext. I can get the slider bar to show in SysPrefs/Display and it also works using my brightness keys. However, the slider doesn't do anything - there is no change in screen brightness. This is not effected by sleep/wake.
Your laptop probably doesn't use the IGPU PWM for backlight control, therefore it is not compatible with AppleBacklight.kext (would not be compatible with IntelBacklight.kext either).
Your only choice is ACPIBacklight.kext + Brightness by bergdesign (for controls on the menu bar, since native controls won't work).