I feel your pain. We have a rescue sheltie (shelties love their own voices at the best of times) who barks incessantly at anything and everything. The number of times I have thought of rehoming as a happy solution could not be counted; I could never do that, though, because she had an awful time before coming to me.
Punitive methods of any sort seem to fail as did training to bark on command. I don't have any great solution, but a couple of small ones.
I walk her as often and much as I can. If I pop in the car to collect a DC, she gets strapped in and comes too. The more mentally and physically tired she is, the more she sleeps and the less she barks.
We've tried valarian, rescue remedy and a dap diffuser with limited success. They may help a little, it's hard to say. I think mostly they've helped as a placebo to me feeling I am doing something to improve things!
Known triggers (DH coming in, doorbell, dinner time, me doing anything that looks a bit exciting, oh dear I could go on!) I pick her up and carry her round for a bit. This seems to shut her up instantly and she seems to be calm if I'm carrying her. I couldn't have done it when I first had her, she'd have bitten me out of fear, but she follows me round adoringly nowadays and so is happy to be picked up. The first four months she was totally silent anyway - she then found her voice in a big way!
When we go out I leave dentastix, treat filled stuff, and shut her in the most insulated, away from neighbours and outside noise room we have. I keep up regular discussions with the neighbours as to whether they hear her and when. Luckily we don't have to leave the dogs alone too often or for too long. If we did I'd have to employ a dog walker.
I don't know if any of this helps. I feel at my wits end sometimes - I just try to minimise it, really.