Two comments from a long-time resident of Japan:
> ... exploring shops in Harajuku and Shibuya with a fellow bag designer
Harajuku and Shibuya are great places to observe the current trends among fashion-conscious people. To learn about the majority, I suggest taking a train out to the suburbs to see what people on the trains and in the shopping malls carry or wear. My guess is that the bags there will be more utilitarian overall.
When I moved to Tokyo in 1983, I was still in college-student mode and carried a backpack at first. I soon noticed, though, that people here didn’t carry them, so I bought something more briefcase-like. Backpacks did become common later, though I can’t remember exactly when that was. I haven’t been paying attention to other carry culture trends, either, so I asked Gemini Deep Research for a report:
https://docs.google.com/document/d/1kl-4WsUxSyE9LrJpKhig0wnT...
However they seem to be producing less of these designs nowadays (change in designer maybe?) though they still have a few which you can checkout at Amazon Japan.