Sorry for the late reply.
It’s so exciting that Shimi will be going on her first international trip soon!
Based on my experience with my daughter, I think the shorter you can make the trip, the better. But our trip last year was 24 hours, so perhaps Shimi will be fine for 16 hours. You also need to consider the possibility of planes being delayed. If there’s a layover, this might mean missing a flight or having your trip end up being much longer than you expected.
For instance, our flight from Narita to Vancouver was delayed and the next flight to my hometown was four hours later. This was when my daughter had a breakdown (understandably, it makes my heart break even now remembering it. She did so well up until then but it was 16 hours of traveling at that point, my poor girl…)
I’ll share this on Twitter (as much as that place disappoints me now, haha) and Bluesky as well to see if other parents can offer some advice.