I need to "release"(without uploading it to App Store) an app, but only for a single device. Can this be done without joining the Apple Developer Program? And without having the device connected or reconnected every fortnight.
Unfortunately, no. You need to join the Apple Developer Program to release any app. When you sign up for the program, you can release the app as a custom app so that it can install app on devices you allow to learn more read here: Distributing Custom Apps