I have family in Japan who I visit every few years and I would definitely recommend having as large of a disposable income as possible. For an opportunity like studying abroad, it would be regrettable to spend every meal at a convenience store to save money, especially with how much good food there is eat and exciting things to see.

The XR and XS are great devices and those prices sound reasonable but in my opinion, I don't think it's worth potentially handicapping your experience in Japan. If you're worried about the S9, maybe a new or second hand iPhone 8 could power you through. The rumored SE2 may or may not be released before your trip as well, which may be worth considering.