Beleive it or not it's how you react to your dog's behavior every moment is actual Training, even how you respond to a certain behavior? You may be encouraging him to bark sending him mixed signals. I remember with my male that if I got excited by something he did that I did not want him to do, the more he did it! So I actually did they opposite I ignored it. I figured out that when I got excited he mistook it for a form of praising. So when he got no response he stopped. Barking is just a form of speech for them, when he is in his guard dog mode, I do praise him. So he understands when he should bark and when he should not. He also has different forms/times he barks. I'm thankful he cannot talk, he would probably talk too much
Good Luck!
It's tricky, this breed is too smart and yes they have to be entertained, exercised and kept busy all the time! Different things work, keep trying.