How can I move Customizer menu item to first level in WP Dashboard?

The function below should help you. It removes the default menu item from the “Apperance” menu item and a new menu item to the Dashboard.

add_action( 'admin_menu', 'fb_customize_admin_menu_hide', 999 );
function fb_customize_admin_menu_hide(){
    global $submenu;

    // Remove Appearance - Customize Menu
    unset( $submenu[ 'themes.php' ][ 6 ] );

    // Create URL.
    $customize_url = add_query_arg(
        'return',
        urlencode( wp_unslash( $_SERVER['REQUEST_URI'] ) ),
        'customize.php'
    );
    // Add sub menu page to the Dashboard menu.
    add_dashboard_page(
        __( 'Customize' ),
        __( 'Customize' ),
        'customize',
        esc_url( $customize_url ),
        ''
    );
}

You should copy this in a small plugin and activate in your installation.