Hi, BillEssley!
If you allow me, I have to mention that automatic updates in WordPress concern only tech releases which mission is to fix some critical WordPress bugs and vulnerabilities. It is not safe and not recommended to skip such updates. You are the exception if you use any tools to deploy projects to remote servers, such as Capistrano or SaltStack, or you may be using a version control system.
Then to completely disable the automatic updates you can try using the following constant in your wp-config.php file.
define( 'AUTOMATIC_UPDATER_DISABLED', true );
Please note that with this directive, you will also disable automatic updates of language packs, themes and plugins, if enabled.
With the command WP_AUTO_UPDATE_CORE, you can control the automatic update of the WordPress core, e.g.
add_filter( 'auto_update_core', '__return_false' );
By default, WordPress will not automatically update your themes and plugins, but this can be easily changed using the auto_update_theme and auto_update_plugin filters:
/**
* Plugin Name: Auto-update Plugins and Themes
*/
add_filter( 'auto_update_theme', '__return_true' );
add_filter( 'auto_update_plugin', '__return_true' );
P.S. I advise you not to add commands to functions.php, because it does not give you any advantages, and this file will no longer be executed if you suddenly decide to change the theme of your site.