If your input devices are anything like mine where 2 is the keyboard then you’re testing the wrong device. The brightness buttons are on a different input device. On my machine it’s /dev/input/event4
I think I had to fix the permissions as well of the backlight device.
I’ve added my user to the video group and created the following udev rule to make it permanent:
SUBSYSTEM=="backlight", ACTION=="add", \
RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness", \
RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness",
Probably not but depends what exactly is missing. On newer kernels it usually shows up for amdgpu but that doesn’t matter.