I agree that it is usually aesthetic or tradition. I had two Border Collies with tails that did some shepherding. I then got a Mini Aussie from a local breeder but I wanted to keep his tail since my BCs ad theirs. When the Mini Aussie got older his tail started curling over his back. By then I did not have sheep for him to herd anymore but I could tell that if his tail got caught in some sticker bushes it could get broken. The BCs tail is used as a tool in herding and stays fairly inline with their bodies thus not very prone to breaking.
I have also heard of BCs tails getting docked in very rural areas to distinguish between wild canines and domestic ones.

I have had a few Siberian Huskies stay with me. The first one I made the mistake of not doing a meet n greet first and the owners had not confirmed the stay before they brought him. He was so hyper and intense that my dogs and a normally very unconfrontational guest dog kept trying to pick a fight with him through my closed sliding glass door. The owners came and got him the next day but never confirmed and never paid.

I have had other Huskies that came from well rounded families and my dogs were seemingly nervous to play with them I don't have an answer for your particular situation but want you to know that perhaps it is the breed not you dog. It sounds like you are doing the socialization correctly. When you want to find a sitter do a meet n greet and meet their dogs too. If possible meet guest dogs that your dog will be getting sat with. Talk to a Siberian Husky breeder or breed club too.

I take care of an Italian Greyhound that has anxiety when her dad is gone. Her dad has a Thunder Shirt for her. I was skeptical at first but as long as she wore her shirt she engaged with people and other dogs. She and her full sized Greyhound sister are repeat customers. One time they came to stay her dad forgot her Thunder Shirt. She would not stop shaking, she kept getting on my dining room table (not something she had done before), and was not being her sweet self. Luckily her dad was still in town and brought her shirt. She stopped shaking and stayed off the table. She was back to lovable self.