wasteoftime this thread may be useful:
@GrapheneOS - Developer options are for development and that option has no connection to our system updater or app repository client.
@de0u - Settings, Apps, "See all ...", Pick "Show system" from the vertical-dots menu, scroll down to System Updater, Disable.
More importantly however, may I ask why you want to disable automatic updates? I've never experienced a more reliable, fast, and (more importantly) unobtrusive update system on a phone. It's truly magnificent.