Basic learning theory says there are only 3 outcomes you can use to change a dog's behaviour: rewards, punishers and nothing at all.
Anti-bark collars are punishing. They have to be. If simple beeps worked to distract a dog from barking reliably then all any of us would have to do is make a beep noise.
They don't work like that.
The beep is a warning to the dog that something is does not like is coming. It is used in the hope that the dog learns that it can change the outcome of that warning by altering its behaviour.
e.g. I hear a beep
I keep barking
My collar vibrates, which I do not like
slowly (hopefully) becomes:
If I hear a beep
And shut up
I can avoid the vibration
Vibrations and citronella ARE things the dog does not like, therefore they are punishers. They might make us feel less guilty than an electric shock, but that's it.
Training dogs with punishments is a tricky minefield in which all kinds of things can go wrong. I am always reminded of a case Sarah Whitehead talks about in her book in which a saluki exposed to something similar became a shaking bag of nerves because she heard any beep in the house as a threat that something unpleasant was coming and never did work out it was just applicable to a specific type of beep in the garden that she could avoid by returning to the house.
Plus, punishment is a moral quagmire.
Training dogs with rewards is a lot simpler, carries less risk and - as seen with guidedog studies - is often quicker and results in more reliable behaviours.
So instead of punishing the dog for barking, reward him for something else.
e.g. the doorbell rings, the dog is encouraged to lie down on his bed, the dog gets a treat. Every single time. With multiple practise sessions to establish a routine.
The Blue Cross has a page on this: www.bluecross.org.uk/pet-advice/how-stop-your-dog-barking
Plus, as pp have said, take a holistic look at the dog's quality of life. A dog that is under exercised, under stimulated, stressed, in pain, worried etc will be much more likely to bark than one that is not. Just like a stressed person is much more likely to snap at you.
He's a schnauzer. He is likely above average intelligence for a dog (so bored easily) and literally designed so that barking is rewarding for him. This is why it is so important to consider the 'negative' breed traits when choosing a breed. Complaining your schnauzer barks is akin to buying a high powered SUV then moaning it needs a lot of fuel.
If that all sounds like too much work, rehome him via a reputable charity so that he has a chance at a life with someone who has the time, energy, etc for him. A well rounded (albeit barky) young schnauzer won't wait for a new home for very long and it's better than another 10 years in a home where he is resented and not getting the training etc he requires.