I have the same exact problem with my maltese. He is worse, I tried everything you did and even did a collar. but it still didn't work and I felt really bad because his neck was so small, he is 6 lbs, and the smallest collar was still big on him. So I took it off.
I just got a tip from a trainer yesterday to lock him in the bathroom when he barks, without saying anything, just put him in there and he can bark all he wants. The moment he stops I give him a treat and he can come out. any bark, he's back in there. I only tried this once the other day and it seemed to work.
I spoil him too much so I am not the alpha dog to him, but when my husband is home, he does not bark at all unless it is absolutely necessary.
