You can just drop your plugins into the addon folder of your Godot projects in that case, you shouldn't need a repository. You might be able to use symbolic links (on Windows) to link all of the addons in your project(s) to a single source on your hard drive.
Also, I am fairly certain you can use git and git sub-modules entirely offline. Services like GitHub and Bitbucket just provide a service for holding the git repository, but the underlying functionality should be built-in to the git command.