Honestly? And this is going to be an unpopular opinion, but follow the advice of your vet even if they suggest having the dog put to sleep.
My parents dog started doing this when he was 2. Vet suggested euthanasia, parents declined.
They have tried dog psychologists, behaviourists, classes, 1:1 training, chemical castration (was awful and upsetting so stopped the medication), had him neutered, different medications, different diets, nothing had worked, he is 10 years old now and bigger and more aggressive than he ever has been.
He snarls, growls, has possession aggression that no one has ever been able to rectify and has bitten my dad a few times.
He is now managed very carefully at home, and has a permanent house lead that we use in an emergency to pull him away but honestly, every time I visit my parents' home with my own dog, I am scared of him. I'm also pregnant, and concerned of visiting their home when the baby arrives.
It is exhausting, and having tried everything they can afford, my parents are now just stuck in this cycle of wondering how he's going to react next time they walk in the room.
He is an extreme case, but a lot of heartache may have been avoided had they listened to the vet in the first place