The guard dog combo I would recommend is one little yapper dog (deterrence) and a chow chow (sneaky and vicious if they still keep coming and enter the premises).
The chow chow does a soft roar like a lion—but you would never hear it above the yapping of the little dog.
Chow chows supposedly were the guard dogs of choice of Genghis Khan.
My first job was at a veterinary office. A chow seemed quiet until it snapped and tore my hand open. It felt like it was on fire.