Above is a small script which you can make executable and add to autostart. It will place an icon in your sys_tray on startup by which you can toggle on/off your existing script (which for this purpose I have referred to as ".screen_brightness.sh"), and I have assumed lives in your home folder. (note the . (hidden file)). Names and file locations of course can be changed to your liking
ksystraycmd --startonshow --keeprunning --tooltip "Adjust Screen" --icon application-pgp-signature ~/.screen_brightness.sh
But only add the above script to autostart as this script will then call your script as required.
Why your script needs root privileges to function I really don't know. If it is owned by you, and not root, does it still function? I can't imagine screenbrightness settings needing elevated privileges TBH.
But i'm not a bash expert by any means. Perhaps others might be able to offer some advice on this?