Adding top level items in the admin menu that link to a post

how to add different redirects to each one of them

You can do an if-elseif-elseif… like this:

if ( condition 1 ) {
    // do something
} elseif ( condition 2 ) {
    // do something else
} elseif ( condition 3 ) {
    // do something else
}

So let’s say you created three top-level admin menu items:

add_menu_page( 'Slider Menu', 'Slider', 'manage_options', 'page_slug', 'function', 'dashicons-images-alt2', 6 );
add_menu_page( 'Page Title', 'Menu Title', 'manage_options', 'page_slug2', 'function' );
add_menu_page( 'Page Title 2', 'Menu Title 2', 'manage_options', 'page_slug3', 'function' );

Then your if-elseif-elseif… would look something like this:

// For the "Slider" menu.
if ( $menu_redirect == 'page_slug' ) {
   wp_safe_redirect( admin_url( 'post.php?post=1883&action=elementor' ) );
   exit();
// For the "Menu Title" menu.
} elseif ( $menu_redirect == 'page_slug2' ) {
   wp_safe_redirect( 'URL here' );
   exit();
// For the "Menu Title 2" menu.
} elseif ( $menu_redirect == 'page_slug3' ) {
   wp_safe_redirect( 'URL here' );
   exit();
}

Btw, I think you should use admin_url() for getting the URL of an admin page. =)