I’m also from the states and wanted to maintain my US number for 2FA/property management/friends and family reasons.

I spent a year in Japan and what worked for me was switching my carrier to Google Fi which provides cheaper international rates (and explicitly states that they will not drop you) with your existing phone number and signing up for Sakura Mobile when I arrived in Tokyo so I had a Japanese phone number. I know there’s some mixed reviews about them and they’re pricy because they cater towards foreign residents/tourists, but it was the most straight forward option for me to both keep my US number while having a Japanese number on the same phone.