Get a default customizeAction text for a section using Customizer JS API

You’re right. There’s currently no default value for the customizeAction param.

You can add your own default value for all controls with something like this in PHP:

add_action( 'customize_controls_enqueue_scripts', function() {
    wp_add_inline_script(
        'customize-controls',
        sprintf(
            'wp.customize.Section.prototype.defaults.customizeAction = %s;',
            wp_json_encode( __( 'Customizing', 'default' ) )
        )
    );
} );

This isn’t in core yet because the default value is actually variable based on whether there is a panel defined. This is something we should figure out and improve in core. See #42635.