Just found a solution with the help from this post:
My approach is slightly different, but based on the info from the previous link:
1 - I created the file /userdata/system/custom.sh (since it was non-existant):
2 - I pasted the content of my script inside:
echo 500 > /sys/class/backlight/intel_backlight/brightness
3 - Saved the contents:
Ctrl+X and then Y
4 - Made the file executable:
chmod a+x /userdata/system/custom.sh
5 - Rebooted the system.
6 - I checked that the solution took effect after rebooting by typing:
The output is 500, success!!
PD: I don't know if it was necessary to remount the filesystem with write permissions before step 1 with the following command:
mount -o remount rw, /
I think that the userdata folder is not read-only by default, so I guess this previous step was not necessary.
I hope this helps anyone with the same or similar situation.
Note that the script can contain any commands that you need to be executed at startup.