The best way to teach a dog to be quiet on command is to teach them to speak on command. That’s the only way you can practice telling them to be quiet.
Putting a behavior on command requires repetition. The only way to practice shushing is to practice barking. Use an accomplice and an appropriate stimuli to train the dog to bark on command.
It’s easy to lure a dog to stop barking: simply present a treat for them to sniff. They can’t bark and sniff at the same time. You needn’t always reward them with the treat, but you should praise them as soon as they stop barking.
An older dog is no excuse. If you don’t want your dog to bark at the door, teach them to “Shush” on command. Challenge extended!
If your reprimand isn’t changing their behavior, then repeating it will only desensitize them. An abrupt and startling reprimand is the most effective. If you do take hold of the dog, be sure to hold them until you can praise them for being good.