I don't know if this will help, but this is what a trainer taught in her puppy classes years ago. It was all positive reinforcement. First, put barking on command. So teach them to speak when you ask them to. Then teach them the quiet command. The idea is that it is easier to learn the quiet command if they know there is a speak command. It also lets them know that there are appropriate times to 'speak', and it is something they should be asked to do. I don't know when this actually sinks in, but its a process. The other thing this trainer explained is many dogs just love to talk. So let them have a bark fest at appropriate times. She suggested to go for a car ride and let the dog bark its fool head off while riding around where it won't bother anybody but you.
So that was her training advise. In my experience, you have to be extremely consistent to get the quiet command to work, and it has to be a positive experience for the dog. I do clicker training to identify when they get what quiet means and jackpot that with treats. But even after they learn it, its very hard to be consistent and you end up yelling at the dog to be quiet and then you are barking too and everything ends up all negative sounding. Corgis don't usually respond as well to that. So... thats my 2 cents. Good luck!
I would also suggest when he barks inappropriately that you refocus him on some activity that you would like such as training sit, shake, roll over etc. Sometimes replacing a behavior with another works well.
Cassie is a barker, too. It's not easy to cope with. No amount of training strategy makes an awful lot of difference, other than putting my hand on her back gently but firmly and being quiet until she quiets down.
Have you thought of trying to find a place to live where the barking won't disturb neighbors? Truth to tell...if I lived in an apartment and a neighbor had a barking dog, I wouldn't like it!
The dog pound warned that Cassie had been relinquished for barking. My lot is almost a quarter-acre, so there's space between dog and audience. But I did tell my neighbors that Cassie was said to bark a lot and asked them to tell me if they were bothered. One of them, who was friendly with me, I would periodically check to find out if she was hearing any barking. Because Cassie is inside most of the time, though, and because she doesn't stand out in the yard and yap, apparently I'm the only one whose ears get deafened regularly.
Yea verily, as we scribble Cassie just tried to launch a barkfest. Why? Who knows? Wanted to get the pup's attention? (It worked.) Heard something out in front? (Again!?!!) Wants me to get off my duff and feed them breakfast? (Probably.) Was able to settle her down quickly and now she's sitting here peacefully again. But it means the human has to break off whatEVER she's doing to quiet the dog down.
Awesome! Good work!