Breed and train working German shepherds. No weapons it would be very difficult if not impossible to defeat a properly trained dog over 50ish pounds. Just a big mean dog your chances go up significantly. You are still gonna be injured
I also trained protection and sport dogs - Mondio and French Ringsport, and PSA. I was a decoy for Mondie and PSA. I have been bitten by a lot of dogs. A particularly assholey Malinois in Mexico opened my face and cost me a tooth.
The biggest challenge is that most people have an instinctual fear of dog bites. That will get you in a lot of trouble as the dog will react to your awkward body language. I have diffused aggressive dogs simply by remaining calm. I have also made dogs bite me just from a look.
It helps to be able to read a dog.
Most dogs will not bite you, however (I'm making this up), if you have to deal with a serious dog coming in, I would try and distract him with a piece of clothing, A jacket waved in front of him could be enough to get him to bite the material and that may diffuse the situation.
The other option, if you feel you're going to be bitten is to wrap the jacket and around your arm and feed it to him and then flip around his back with your legs on each side. You will be able to get some control this way. If you can handle the pain then jam his mouth - dogs cannot bite if you grab their lower jaw - and when he releases grab as much scruff as you can and pull back to more or less immobilize him. Eventually he will calm.
If he refuses to let go, then you have to get medieval on him. Hard as you can, life and death, you're going to scoop his eye out with your thumb. A dog locked into drive will not feel much pain, but no one can take pressure in the eye.
Coming from the working dog world would you rather take the 50 lb Mali who loves it or the real molassers, old neos, boerboels etc?
Our club was known for training a lot of molossers and I saw many of the early Corsos, Presas, Bandogs, Boerboels, and new breeds like Donovan Pinschers; as well as Malinois, Dutch Shepherds, and GSDs.
The only dog to knock me down on a long send was a 45 lb Donovan Pinscher. Speed matters. The early Corsos and Presas were shitty. Poor nerves. Boerboels were athletic, but not great.
I don't think there was a Neo I couldn't outrun.
I would say that the only dogs that scared the shit out of me were LGDs from Eastern Europe and Asia.