I'm not sure exactly what you have done but one of the scripts had a reboot plist in it, do you have anything like that under /Library/LaunchDaemons/ ?

Also if you did any tests in your script using the 'pmset' command or 'systemsetup' command, pay close attention to your energy saver settings. You can set a shutdown/reboot schedule with those commands. Look specifically at the Schedule... button in the GUI on your energy saver settings.

Would this be it?

Yup... you are issuing a reboot command with that launchd job

how do I stop this?

I would need the exact path and name of the file you found but it should go something like this

“launchctl unload /<your file location>/org.your.rebootdelay.plist”

The file name was taken from the plist file you attached. If it is in LaunchDaemons you may need to run with sudo

Is it also suitable to delete the file?

Once you’ve unloaded, yes

Try without .plist

You can delete the file and reboot, but shouldn't be necessary...I saw the label in your plist above but could I see the actual file in a Finder window...maybe it's named something differently than org.your.rebootdelay.plist