I have 5 Savannah cats and 2 Burmese cats. I had 2 GSD's living with all of them until recently until my 14 year old male GSD passed away.
My house is a mish mosh mash up of when everyone joined, so there have definitely been adjustment periods. Dogs getting used to each other, dog getting used to a cat, 2, 5, 7 cats. Savannah cat kicking dog bootay.. dog taking it, male Burmese cat falling in love with female GSD and practically pulling a Glenn Close in Fatal Attraction with this poor dog daily, literally chasing her from room to room to roll over her nose and fall over her head.
Like drops in the water bowl... these are the days of our lives...
We have 1 big girl Savannah that hates everyone, dog, cat, and person, except me. I have worked with some real doozies. My girl GSD came to me because she'd bit a child and was dumped in a shelter. I've had no issue with her. I don't put her in situations where that will ever
happen again, but she's been work. I have another Savannah that was bought by someone and then basically abandoned in an attic for a year of his life, sick with a parasite that cause horrific diarrhea, and required chemo meds to treat. He didn't get treatment till I found him and got him out of the attic. He was a nightmare. He terrorized the other animals, dogs, cats, us too. Plain mean..more Serval than domestic cat. It took me over a year, but today, you wouldn't recognize him, physically or emotionally/mentally. He head-butts everyone, dogs and cats too for love, cuddles with everyone when sleeping and no longer fears the litter box (it held pain to use it).
To make it short, all of pets are purebred, but they didn't come from the hottest of situations and all but 4 have required rehabilitation and medical intervention. My newest girl.. the big Savannah F-2, I've had almost 2 years, I just stopped bleeding upon interacting with her 7 months ago. My husband and son, cannot even think about getting near her without hawk gloves on, and her claws do get through those. I can now pick her up for 5 minutes at a time, 3 times a day, and get licks on my cheek every time I walk into her room. It's slow, but it's progress.
Many people have told me to give up and put her down as being "too" Serval, and I just can't do it. I'm in it, and she won't be put down for simply being what she was bred to be, then not properly raised. I got her at 2 years of age, Savannahs mature at 3, so I hope I'm helping. I'll keep on keepin' on.
It sounds like you're doing everything you can humanely do, and then even doing more.
Keep us updated, but for what it's worth, sometimes it takes alot longer, and then sometimes, it doesn't matter how much time you put in, it will never be enough. I hope it works, because that dog would be so lucky to have you.