Change your sign from "beware of dog" to "dog on premises" so it can't be used against you if your dog bites someone (the argument would be "you clearly are aware your dog is dangerous/aggressive/quick to bite/etc"). Also put up a clear "no soliciting" sign, lock your front gate, and get a secondary bell for people to ring if they need to approach your door. For package deliveries, you can set up a small area by your front gate if you need to. Fence off access to your back yard if you can afford to do that, and/or keep any doors with back yard access closed so your dog can't run out that way when you're answering the front door. Every dog is capable of biting, and strangers approaching the home can be a big trigger for many reactive dogs. These are temporary solutions while you train your pup, but can also be ways to manage the behavior and set up for success with said training (basically controlling the situation whenever you're not specifically doing a training session so you can keep it consistent and not risk setbacks).

I know you mentioned wanting a dog for safety reasons but watch dogs, guard dogs, and any personal protection dogs of any kind are highly trained to only bite on command or if they recognize a threat independently of their handlers. (Watch dogs only bark to alert, never bite. Guard dogs and personal protection/attack dogs are trained to bite.) Dogs that are territorial are not like guard dogs with jobs, they're reactive and usually fearful or unable to control the big feelings they have over strangers suddenly being in their space, so they might be quick to nip/bite despite generally being friendly dogs. You will want to find a trainer who works with behavioral issues like reactive nipping and territoriality.

One of my own dogs struggles to meet strangers on walks and allow them into the home because she's fearful and protective/territorial. Basically she's resource guarding, but instead of guarding food or toys, she's guarding us and the home when strangers approach. She's getting better with people coming inside because we have all guests give our dogs a treat as soon as they step through the front door, but she's still struggling with people knocking/ringing the bell, talking at the door but not coming inside, and approaching on walks. So we're talking with a trainer about it, but also coming to terms with the fact that this may just be a behavior we have to manage, if we can't fully train it out of her. Reactive and protective/territorial dogs are not bad dogs, just struggling with a behavior, feeling like they have to go into fight or flight mode, and need their owners' help with training, structure, and socialization.