OP, I think you’ve already gotten some excellent advice. Be consistent, you can turn this around.
Some things I feel need to be addressed:
Exercise - I haven’t seen this discussed much. Is the dog getting enough? If not, run him into the ground. This works best with anxiety and destructive behavior, but it can’t hurt with your situation. Tired dogs are happy dogs.
Physical domination vs beating - There’s a difference. I’ll just assume people are being sloppy with their word choice and aren’t actually a bunch of deadbeat, animal abusers. You need to be firm, consistent, and dominant. Extra emphasis on consistent.
Shooting - People advocating for this need to chill TF out. We get it, you’re a big, strong man.
End game - OP, if the situation becomes untenable, please contact a breed specific rescue. Be honest about the situation, there’s no shame in saying “this dog needs more than I can give it right now.” Someone wants that dog. PM me if you want; I’m happy to help you find a rescue.
Good luck and happy new year!