I don't have any answers for you, but some things to consider.
Our GSD lives in the house with us. At night and when we are gone he is inside. We used to crate him, but a few months ago we decided he was trustworthy enough that we could leave him alone. HOWEVER, we raised him from a pup. Bringing an adult into the home is different.
Is the outside area where you would leave him fenced? We've never tied our dog down, and I think the general consensus here would be that doing so is not a good ideal.
You already have five dogs, so if you do this, you have to be very careful that the GSD you pick is not dog aggressive. And if you look around Ed's site, you'll find articles about introducing a new dog to your pack, in this case a much larger dog. You have to do that carefully.
I don't know how long it takes a GSD to begin feeling protective of its new home. That is something to think about.
You can find brokers who sell adult dogs. If the broker is any good the dog will not be an unknown quantity, there will be some history. You could also look for GSD rescue groups. Many of them foster their rescues until they are placed, and so may be able to give you an idea of how the dog is with children and other dogs.
Something else. Let's pretend the robberies are not an issue. Do you really want another dog? You have small dogs, do you really want a big one? A dog that will shed as much as your five other dogs put together? OK, that MAY be an exaggeration, but you see my point. GSDs shed, and they shed a lot. Do you want a dog that won't be satisfied with the amount of exercise he can get running around the house? Assuming, of course, that you want a dog as big as a GSD running anywhere inside.
As far as a deterrent is concerned, I think a GSD could provide that. Mine lights up whenever anyone comes onto the property, and he can be heard from the street. I'm convinced that anyone who was up to no good would turn around and go somewhere else once they heard him barking and growling. Then again, our next door neighbors have two little Italian Greyhounds. They light up whenever they see or hear a stranger, or another dog in the neighborhood barking. They attract a lot of attention. Would anyone hear your 5 small dogs barking?
Finally, I love GSDs. I think they make excellent companions and are a reasonable deterrent to unarmed crimes, such as home break ins. However, bringing any large dog into your pack of small dogs is something you will need to manage. Maybe you'll get lucky, but I think it unlikely that you'll find a dog that fits right in without a fair amount of work on your part. So if this is what you want to do, spend some time on Ed's site reading the articles. Whatever you do, you want to do it with your eyes wide open and fully understanding what you are getting into.