Hi @ruenel,
Somehow I kept thinking there should be a way to manipulate the default WP menu to achieve this. Long story short: there is!
Turns out the 'wp_get_nav_menu_items' filter is perfect for this, as it let you manipulate the nav-menu, including the items!
Posting the full custom code below. I don’t think any customer should expect customer support to come up with a custom like this. I certainly didn’t. But since I’ve been able to figure it out, I would like to share the code, as it may help others.
Feel free to criticize the code tough, any ideas that might make this code even better I would be happy to incorporate and share here, for others to benefit from!
add_filter('wp_get_nav_menu_items', function ($items, $menu, $args) {
    
	//Dont apply to admin or ajax calls
	//if (is_admin() || wp_doing_ajax()) return $items;
	if (wp_doing_ajax()) return $items;
	
    //Set allowed taxonomy and post-type
    $allowed_taxonomies = ['categorie']; 
    $post_type = 'aanbod';
    $new = [];
    $uid = 10000;
    
    //Little helper to count the number of children per parent
    $children_count = [];
    foreach ($items as $it) {
        $pid = (int) $it->menu_item_parent;
        if ($pid) {
            if (!isset($children_count[$pid])) $children_count[$pid] = 0;
            $children_count[$pid]++;
        }
    }
    foreach ($items as $item) {
        
		//Copy existing item values to new var
        $new[] = $item;
        // Only coninue if item is a taxonomy
        if ($item->type !== 'taxonomy') continue;
        
		//Only continue if the taxonomy of this item is one of the 'allowed' list
		$taxonomy = $item->object;
        if (!in_array($taxonomy, $allowed_taxonomies, true)) continue;
		$pid = (int) $item->ID;//Current postID
        $term_id = (int) $item->object_id;//Current termID
		
		//Get all posts for the defined post_type that have this term
		$q = new WP_Query([
            'post_type'      => $post_type,
            'post_status'    => 'publish',
            'posts_per_page' => -1,
            'orderby'        => 'date',
            'order'          => 'DESC',
            'no_found_rows'  => true,
            'tax_query'      => [[
                'taxonomy'         => $taxonomy,
                'field'            => 'term_id',
                'terms'            => $term_id,
                'include_children' => false,
            ]],
            'fields'         => 'ids',
        ]);
		$post_count = !empty($q->posts) ? count($q->posts) : 0;
		
		//Add all found posts as submenu-items
        if (!empty($q->posts)) {
            foreach ($q->posts as $post_id) {
                $uid++;
                $virtual = (object)[
                    'ID'               => $uid,
                    'db_id'            => $uid,
                    'menu_item_parent' => $pid,
                    'object_id'        => (string) $post_id,
                    'object'           => $post_type,      
            		'type'             => 'post_type',     
            		'type_label'       => 'Post',
                    'title'            => get_the_title($post_id),
                    'url'              => get_permalink($post_id),
                    'classes'          => ['menu-item-object-'.$post_type],
                    'status'           => 'publish',
					'menu_order'		=> $uid, 
                ];
                $new[] = wp_setup_nav_menu_item($virtual);
            }
        }
    }
    return $new;
	
}, 999, 3);