Normally when you have installed a XFCE-Desktop image for the Pinebook A64 you could set the display brigthness with the following pkexec command to a readable higher level - like I did in the past (possible brightness values are 1-10 - on startup this is only set to 2):
pkexec /usr/sbin/xfpm-power-backlight-helper --set-brightness 8
read-out command for the current value:
pkexec /usr/sbin/xfpm-power-backlight-helper --get-brightness
but on standard CLI-install pkexec and xfpm-power-backlight-helper are missing, so to use this command-line (in /etc/rc.local) you have to install these 2 packages:
sudo apt install xfce4-power-manager pkexec