German Shepherd Beagle mix puppies
First off I should say I train dogs a little and have my own personally trained GSD, who is very well behaved and happy.
What I’d try first is to alert him every time someone is at the door by asking “Is someone there?”/ “Who’s there?”. Use a happy voice, so s/he’ll be encouraged to yap/bark in response to you. After a couple of days, his instincts should start to kick in, even just a little. I often “think” myself to a solution, when I train dogs, but sometimes I don’t have the imagination/experience to predict the dogs behavior in complicated situations, so I use YouTube*.
Of course this requires a little training, but less than you’d think, because protecting the flock is natural to dogs. When it comes to training I can highly recommend finding a training style that befits your dog - not just you - and looking for dog trainers on youtube, who use these techniques. Just choose the specific exercise you want to teach your dog and watch a few different videos and try them until s/he responds to the exercise.
Little tip: the more eye contact and communication you have with your dog during training and leisure time, the more the dog reacts to your commands. Also smile. If my dog sees I’m happy, he knows it means good things (play time) and listens really well. When he is naughty I use vocal and facial expressions to show that I am mad, but when he stops, I smile happily and pet him. This means he gets immediate reward for stopping bad behavior.
After my German Shepard was a year old, he started growling/barking, if people got too close to our door or rang the doorbell. This is a good and healthy sign: He is alerting me to the presence of “intruders” and it is a natural instinct. It should not be curbed, but forged into what you need. I had to teach him, to come to me, be quite and wait until I have checked it out. Then I tell him what the disturbance is (ex. friends, delivery, neighbors etc). This way he learns that I have things under control and I can see he reacts well to this. To become a proper guard dog, I need to teach him how to protect, but I don’t have the equipment yet (special sleeves etc).