Working as a W2 contractor for a US company in Japan

I am a few weeks out from entering Japan on a student visa to learn Japanese (yay!!). I am aware that my visa allows me to work around 28 hours per week. I am currently living in the US as a permanent resident and working remotely as a contractor through an agency as a resource for a large US company. I’d like to suggest to the company that when I move they keep me on part-time working ~20 hours per week. I’d therefore be working as a contractor via US based agency for a US company. I think there’s a decent chance of them accepting, but I’m wondering if there’s any other paperwork I need to file or be aware of? Or has anyone else had this situation and have any guidance or advice?