Django Many-to-Many Model relationship

I am writing a newsletter app for Django. The app handles subscriber enrolling, newsletter rendering and sending. I am stuck with my lack of understanding of databases with respect to the Many to Many relationship and the 'through' model.

My models.py has a subscriber model and a newsletter model and a 'through' model that binds them. There is also an email read and date_read field in the 'through', which will be updated the first time that the subscriber opens the email and allows images (not guaranteed I know).

One subscriber can have many newsletters and one newsletter can have many subscribers.

Here is an abbreviation of my models.py:

    class Subscriber(models.Model):  
        first_name = models.CharField(max_length=30)  
        #etc..  
        newsletters = models.ManyToManyField('Newsletter', through='NewsletterEngagement')  
        
            def __str__(self):  
                return '%s %s %s %s %s %s' % \  
                    (self.pk,  
                    self.first_name,  
                    #etc..,  
                    )  

    class Newsletter(models.Model):  
        title = models.CharField(max_length=200)  
        #etc..  
        subscribers = models.ManyToManyField('Subscriber', through='NewsletterEngagement')  
        
        def __str__(self):  
            return '%s %s %s %s' % \  
                (self.pk,  
                 #etc..,  
                 )  

    class NewsletterEngagement(models.Model):  
        subscriber = models.ForeignKey(Subscriber, on_delete=models.CASCADE)  
        newsletter = models.ForeignKey(Newsletter, on_delete=models.CASCADE)  
        read = models.BooleanField(default=False)  
        read_date = models.DateField(null=True)  
        
        def __str__(self):  
            return '%s %s %s %s' % \  
                (self.subscriber,  
                 self.newsletter,  
                 self.read,  
                 self.read_date,  
                )  

The model seems to work quite well as follows:

    >>> s1 = Subscriber.objects.get(pk=1)  
    >>> s2 = Subscriber.objects.get(pk=2)  
    >>> n1 = Newsletter.objects.get(pk=1)  
    >>> n2 = Newsletter.objects.get(pk=2)  
    >>> n3 = Newsletter.objects.get(pk=3)  

    >>> NewsletterEngagement(subscriber=s1, newsletter=n1).save()  
    >>> NewsletterEngagement(subscriber=s2, newsletter=n1).save()  
    >>> NewsletterEngagement(subscriber=s2, newsletter=n2).save()  

And the following queries work as expected:

    >>> n1.subscribers.values()  
    >>> <QuerySet [{'id': 1, 'first_name': 'XXXXXXS1', #etc.. }
                   {'id': 2, 'first_name':'XXXXXXS2', #etc..} 
                  ]>  

    >>> n2.subscribers.values()  
    >>> <QuerySet [{'id': 2, 'first_name': 'XXXXXXS2', #etc.. }   
                  ]>  

    >>> s1.newsletters.values()  
    >>> <QuerySet [{'id': 1, 'title': 'Newsletter One', #etc..}]>  

    >>> s2.newsletter.values()  
    >>> <QuerySet [{'id': 1, 'title': 'Newsletter One', #etc..},  
                   {'id': 2, 'title': 'Newsletter Two', #etc..}  
                  ]>  

My questions are:

1. Is the model fit for its purpose and if not, how could I improve it?
2. I don't know how to get all subscribers who have not been sent a newsletter (n3 above)?

I hope that I have made my questions clear and I look forward to your suggestions.