Move plugin-settings to ‘Settings’-menu in the admin

You seem to be making life complicated. Here’s the code you need;

function menu_shuffle() {
  remove_menu_page( 'plugin-page' );
  add_submenu_page('options-general.php','Page Title', 'Menu Label','manage_options', 'plugin-page' );
};

add_action( 'admin_menu', 'menu_shuffle' );

plugin-page can be found by mousing over the existing menu label and getting the part after the ?page=in this case ?page=plugin-page

The WordPress function definition for add_submenu_page() https://developer.wordpress.org/reference/functions/add_submenu_page/ includes a list of all the menu page slugs (and you need the ‘.php’).

so in the example above for Yoast (bless its pointy little head and use Hide SEO Bloat)

function menu_shuffle() {
  remove_menu_page( 'wpseo_dashboard' );
  add_submenu_page('options-general.php','Yoast SE0', 'SEO','manage_options', 'wpseo_dashboard' );
};

add_action( 'admin_menu', 'menu_shuffle' );