I got my phone service from Mobal. I needed to upload a copy of my passport, but the process was pretty painless - https://www.mobal.com/japan-sim-card/
My plan has unlimited data, but it gets throttled if I use more than 7GB. Last month I went over that, but the data still worked. It was too slow for video, but was fine for social media, which is what I mostly use it for.
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