Broadcasting to all recipients of a message

Hello everybody,

I am a little stuck at the moment. So I really hope you can help me out :-)

I am building a little private message system for my Laravel app.

I would like to notify my users on the Event MessageCreated.

This is my event:

<?php

namespace App\Events;

use Illuminate\Broadcasting\Channel;
use Illuminate\Queue\SerializesModels;
use Illuminate\Broadcasting\PrivateChannel;
use Illuminate\Broadcasting\PresenceChannel;
use Illuminate\Foundation\Events\Dispatchable;
use Illuminate\Broadcasting\InteractsWithSockets;
use Illuminate\Contracts\Broadcasting\ShouldBroadcast;
use Cmgmyr\Messenger\Models\Message;

class MessageCreated implements ShouldBroadcast
{
    use Dispatchable, InteractsWithSockets, SerializesModels;
    
    public $message;
    
    /**
     * MessageCreated constructor.
     *
     * @param \App\Message $message
     */
    public function __construct(Message $message)
    {
        $this->message = $message;
    }
    
    /**
     * Get the channels the event should broadcast on.
     *
     * @return \Illuminate\Broadcasting\Channel|array
     */
    public function broadcastOn()
    {
        return new PrivateChannel('conversation.'.$this->message->user_id);
    }
}

This is my notification:

<?php

namespace App\Notifications;

use Illuminate\Bus\Queueable;
use Illuminate\Notifications\Notification;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Messages\BroadcastMessage;
use Illuminate\Support\Facades\URL;
use Illuminate\Support\Facades\Lang;

class MessageCreated extends Notification
{
    use Queueable;
    
    protected $message;
    
    /**
     * Create a new notification instance.
     *
     * @return void
     */
    public function __construct($message)
    {
        $this->message = $message;
    }
    
    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed $notifiable
     *
     * @return array
     */
    public function via($notifiable)
    {
        return ['mail'];
    }
    
    /**
     * Get the mail representation of the notification.
     *
     * @param  mixed $notifiable
     *
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toMail($notifiable)
    {
        
        return (new MailMessage)->subject('...'));
    }
    
    /**
     * Get the array representation of the notification.
     *
     * @param  mixed $notifiable
     *
     * @return array
     */
    public function toArray($notifiable)
    {
        return [//
        ];
    }
}

This is my event subscriber

<?php

namespace App\Listeners;

use App\Message;
use Illuminate\Queue\InteractsWithQueue;
use Illuminate\Contracts\Queue\ShouldQueue;
use App\Events\MessageCreated;
use Cmgmyr\Messenger\Models\Thread;
use Illuminate\Support\Facades\Auth;
use App\Notifications\MessageCreated as MessageCreatedNotification;
use App\User;

class MessageEventSubscriber
{
    
    /**
     * MessageEventSubscriber constructor.
     */
    public function __construct()
    {
        //
    }
    
    /**
     * @param \App\Events\MessageCreated $event
     */
    public function handle(MessageCreated $event)
    {
        $participants = Thread::findOrFail($event->message->thread_id)
            ->participantsUserIds();
    
        
        foreach ($participants as $participant) {
            if ($participant != $event->message->user_id) {
                $user = User::findOrFail($participant);
                $user->notify(new MessageCreatedNotification($event->message));
            }
        }
    }
}

I am also working with Laravel Echo:

window.Echo = new Echo({
    broadcaster: 'socket.io',
    host: window.location.hostname + ':6001'
});

window.Echo.private('conversation.'+App.user)
.listen('MessageCreated', (e) => {
    console.log(e);
});

Well - the Mails are being sent without problems. But now I'm a bit stuck on how to send a broadcast message to all the users in the recipients list. And of course I need to do this the most secure way.

I read the Laravel docs the whole day, but unfortunately I did not get a correct solution yet.

Your help is highly appreciated!