If you connect your functions with proper hooks your submenu page will be displayed after your post type and taxonomy. Take a look at my example:
/**
* Register event post type
*
* Function is used by init hook
*/
function wpse_288373_register_event_post_type() {
$labels = array(
'name' => __( 'Events' ),
'singular_name' => __( 'Event' ),
'add_new' => __( 'Add new' ),
'add_new_item' => __( 'Add new' ),
'edit_item' => __( 'Edit' ),
'new_item' => __( 'New' ),
'view_item' => __( 'View' ),
'search_items' => __( 'Search' ),
'not_found' => __( 'Not found' ),
'not_found_in_trash' => __( 'Not found Events in trash' ),
'parent_item_colon' => __( 'Parent' ),
'menu_name' => __( 'Events' ),
);
$args = array(
'labels' => $labels,
'hierarchical' => false,
'supports' => array( 'title', 'page-attributes' ),
'taxonomies' => array(),
'public' => true,
'show_ui' => true,
'show_in_menu' => true,
'show_in_nav_menus' => false,
'publicly_queryable' => true,
'exclude_from_search' => false,
'has_archive' => true,
'query_var' => true,
'can_export' => true,
'rewrite' => array('slug' => 'event'),
'capability_type' => 'post',
);
register_post_type( 'event', $args );
}
add_action( 'init', 'wpse_288373_register_event_post_type' );
/**
* Register submenu
*
* Function is used by admin_menu hook
*/
function wpse_288373_register_submenu_page() {
add_submenu_page('edit.php?post_type=event', 'Event settings', 'Settings', "manage_options", 'settings', 'wpse_288373_event_settings', '');
}
add_action('admin_menu', 'wpse_288373_register_submenu_page');
/**
* Register submenu page
*
* Function is used by add_submenu_page function
*/
function wpse_288373_event_settings() {
return;
}