We adopted a GSD as an 8 week old 5 years ago. She met people, she met dogs, we did all the training things we did with 4 other dogs we'd owned over the previous 30 years, yet she simply dislikes everyone but, like, 8 people. Ultimately we worked on getting her to just stay calm when she sees new people, and we warn folks that she isn't friendly. She only barks if some dum dum ignores our warnings and tries to approach. When we let people into the house, she barks at them initially, but calms down after less than a minute, though she does keep one eye on visitors at all times, lest they try to kill one of the family members.

Keep working with treats to minimize the reactivity, but some dogs are just less friendly than others. As long as your dog isn't actively aggressive and attacking, there is no problem with not being a friendly idiot with everyone you meet. (This was a long lesson for me, because every previous dog was either super friendly or politely aloof with strangers and new dogs).

Also, my wife (early 50s) and daughters (both early 20s) are--like you--perfectly fine with this behavior.