ksystraycmd --startonshow --keeprunning --tooltip "Adjust Screen" --icon application-pgp-signature ~/
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 ""), 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

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?