使用隐藏菜单创建自定义帖子类型

I'm creating a plugin in WordPress. I've created a custom post type called "relationlink". With some custom capabilities. Here is my code for the creation of the custom post type:

register_post_type( 'relationlink', array(
    'labels'             => ...an_array_with_label...,
    'description'        => __( 'Description.', 'ovasconnect_churches' ),
    'public'             => false,
    'publicly_queryable' => false,
    'show_ui'            => true,
    'show_in_menu'       => false,
    'query_var'          => true,
    'rewrite'            => array( 'slug' => 'relationlink' ),
    'capabilities'       => array( 
        'create_posts'           => 'edit_relationlink',
        'edit_post'              => 'edit_relationlink',
        'read_post'              => 'view_relationlink',
        'delete_post'            => 'delete_relationlink',
        'edit_posts'             => 'edit_relationlink',
        'edit_others_posts'      => 'edit_relationlink',
        'publish_posts'          => 'edit_relationlink',
        'read_private_posts'     => 'edit_relationlink',
        'delete_posts'           => 'delete_relationlink',
        'delete_private_posts'   => 'delete_relationlink',
        'delete_published_posts' => 'delete_relationlink',
        'delete_others_posts'    => 'delete_relationlink',
        'edit_private_posts'     => 'edit_relationlink',
        'edit_published_posts'   => 'edit_relationlink',
    ),
    'has_archive'        => false,
    'hierarchical'       => false,
    'menu_position'      => null,
    'supports'           => array( 'title' ),
    'map_meta_cap'       => false
));

I'm not displaying it in the menu. And created my own menu with the link to edit.php?post_type=relationlink like this:

add_action( 'admin_menu', function() {
    add_menu_page('My PluginName', 'My PluginName', 'edit_relationlink', 'parentslug', [$this, 'informationPage'] );
    add_submenu_page( 'myparentslug', 'Information', 'Information', 'edit_relationlink', 'parentslug', [$this, 'informationPage'], '', '1' );
    add_submenu_page( 'myparentslug', 'RelationLinks', 'RelationLinks', 'edit_relationlink', 'edit.php?post_type=relationlink', null, '', '2' );
});

This works just fine. I can edit the post_type. But when i'm clicking on Add new, my permission is denied: Sorry, you are not allowed to access this page.

The strange thing is, when i set show_in_menu to true its works just fine.

I've also tried to set show_in_menu to parentslug but then i can edit but also can't add.

My Question

Has anyone an idea how to show only a link in the menu to edit, in my custom menuitem, but so, that i can add items aswell?

(Edit)

Just found out, that if you create a menu item that has the url post-new.php?post_type=relationlink it all works just fine. But i don't want it in my menu...

It seems to be needed in the menu to add a custom post type. Any thought on that?