@ttobe2 Former vet tech and dog trainer; it's okay to consider euthanasia.
Of course, it's the last option to be considered, but OP, you really sound like you've been doing everything you can for your dog. People on this thread might jump on you for bringing the subject up, and will question you endlessly but they can't know your situation. You, your vet, and other professionals you've consulted are the ones that can understand your dog and situation.
I am a bit worried that as your child gets bigger, they may become a target for aggressive outbursts if they reappear. Kids tend to bring that out in dogs by pulling on them, cornering them, getting too close to high-value objects, etc. If your dog is willing to attack its terrier housemate, I'd be willing to bet it may bite your child. Since it can happen in seconds, you may not be able to intervene, and is something to consider.
For bigger or medium dogs, 8 years is a decent lifespan, as they usually make it between 10 to 14 years at most. So if you've given your dog everything you can, be at peace with whatever you decide. It's a difficult situation to be in, and I can tell you care for the dog.
Agree also that it sounds like there might be issues with the husband, but everyone's saying it so I'll leave it at that.