This can always be a difficult situation because of your desire to do everything you can for your dog. Afterall, you rescued him from a shelter to provide him a happy life. However, there are times when the dog does not fit a household. It sounds like you know his personality and take responsibility for his behavior by trying to avoid problem situations. It also seems as though you have taken the steps to social and train him. However, there are some dogs that cannot be broken of this behavoir. Despite what you may see on TV, some dogs cannot be conditioned. The good news is that you live in a rural area which limits exposure to people stopping by, door to door sales, etc. However, all it takes is a bite to the wrong person to create a legal issue for you. Especially, now that this dog has a bite history. So, if you are looking for someone to tell you it is okay to part ways with this dog, it is okay to do so. I am a big believer that dogs should enhance our lives, not stress them out. You have done a great service to this dog for trying to provide a good home for him. But if all the training and work you have put into him has not helped, he may never improve. Now, if you are set on keeping him (which is okay too, I don't what to sound too soapboxy here), then make sure he is neutered and consider trying a different trainer. You may also discuss some anxiety medications with your vet. This could take the edge off and help his demeanor with strangers. There is also a product called DAP (dog appeasing pheromone) which comes in a collar, spray and plug in aerosolizer. This emits a pheromone that calms them. I would not expect miracles with any of these options, but it may be worth a try. I hope this helps and gives you direction.