WP Query to display events and custom post type in a set order

I have implemented the following code to my them to achieve the custom ordering for featured events (first), regular events (second), and notices (third).

New additions to the code include:

1. Merged the posts using array_merge like this: $merged_posts = array_merge( $featured_events->posts, $future_events->posts, $notices->posts );
2. Ran a foreach to set a custom post order for featured events, regular events, and custom post type using get_post_meta and $post->post_type === 'tribe_events' )
3. Sorted by custom order using usort( $merged_posts, function ( $a, $b ) { return $a->custom_order <=> $b->custom_order; });
4. Used $merged_post_ids = wp_list_pluck( $merged_posts, 'ID' ); to select the post IDs for the final query.
5. Added 'post__in' => $merged_post_ids and 'orderby' => 'post__in' to the final query.

The final working query is below for reference.

// Notices and Events query
$featured_events_args = array(
  'post_type' => 'tribe_events',
  'post_status' => 'publish',
  'posts_per_page' => -1,
  'meta_query' => array(
    array(
      'key' => '_tribe_featured',
      'value' => '1',
      'compare' => '=',
    ),
    array(
      'key' => '_EventStartDate',
      'value' => current_time('Y-m-d H:i:s'),
      'compare' => '>=',
      'type' => 'DATETIME',
    ),
  ),
  'orderby' => 'meta_value',
  'meta_key' => '_EventStartDate',
  'order' => 'ASC',
);
$future_events_args = array(
  'post_type' => 'tribe_events',
  'post_status' => 'publish',
  'posts_per_page' => -1,
  'meta_query' => array(
    array(
      'key' => '_tribe_featured',
      'compare' => 'NOT EXISTS',
    ),
    array(
      'key' => '_EventStartDate',
      'value' => current_time('Y-m-d H:i:s'),
      'compare' => '>=',
      'type' => 'DATETIME',
    ),
  ),
  'orderby' => 'meta_value',
  'meta_key' => '_EventStartDate',
  'order' => 'ASC',
);
$notices_args = array(
  'post_type' => 'notice',
  'post_status' => 'publish',
  'posts_per_page' => -1,
  'orderby' => 'date',
  'order' => 'DESC',
  'tax_query' => array(
    'taxonomy' => 'notice-type',
    'field' => 'term_id',
    'terms' => array( 48, 49, 50, 51, 60 ),
  ),
);
$featured_events = new WP_Query( $featured_events_args );
$future_events = new WP_Query( $future_events_args );
$notices = new WP_Query( $notices_args );
$merged_posts = array_merge( $featured_events->posts, $future_events->posts, $notices->posts );

foreach ( $merged_posts as &$post ) {
  if ( get_post_meta( $post->ID, '_tribe_featured', true ) == '1' ) {
    $post->custom_order = 0;
  } elseif ( $post->post_type === 'tribe_events' ) {
    $post->custom_order = 1;
  } else {
    $post->custom_order = 2;
  }
}

// Sort by Custom Order
usort( $merged_posts, function ( $a, $b ) {
  return $a->custom_order <=> $b->custom_order;
});

$merged_post_ids = wp_list_pluck( $merged_posts, 'ID' );

$final_query = new WP_Query(
  array(
    'post_type' => array(
      'tribe_events',
      'notice'
    ),
    'post__in' => $merged_post_ids,
    'posts_per_page' => 4,
    'orderby' => 'post__in',
  )
);

Thanks to @alexandrucarjan for the assistance, and for leading me toward a solution.