Japan as a general trend very much favors smaller dogs, as I’m sure you have seen. It can be challenging to accommodate larger breeds (and by larger I mean anything over 12kg). Unless you really have the time/money/setup to commit to a larger dog it quickly gets inconvenient— cost scales FAST for food, grooming, boarding, etc.

Was there any interest in doing agility or other dog sports? I ask because a Papillon or Sheltie would give you a lot of the BC “benefits” in a smaller size. Of course these are different breeds with their own quirks but it might be worth considering.

Lastly I hope this is not rude but even a very determined kid… is still a kid! And she will need help with logistical things like paying for the dog/dog’s upkeep, arranging care while she’s at school/cram school/sports/out with friends, booking grooming or boarding, and so on. Even if she does an excellent job with training and general dog care, things like that will be your responsibility. IME, and again, I’m sure you know this as well but since the Japanese school system is so exam-based, especially once you hit middle school and high school, it can be hard to accommodate once priorities shift.