Trying to pick a location

We’ve been going back and forth between Argentina, Japan, Vietnam, and Spain.

The issues are:

• my fiancé has already been to Japan (I wanted to go somewhere new for both of us, but his previous experience could be highly beneficial. I believe he even speaks some Japanese)

• both of us have anxiety in large crowds (so we’re thinking Spain may not be ideal)

• Argentina (the idea here is a week long train ride down south and spending another week near the glaciers which I don’t think is ideal the time of year we want to go)

• I have to be gluten free and we both don’t know if Vietnam is plausible with this issue

Any advice would be appreciated! Or if another location might fit our criteria I’d love to hear it. Thanks!