Hi @reefer! Gabby here from the Community Engagement team at Spocket.
Dropshipping can be done anywhere so there shouldn't be any problems with setting up your online store that caters to mostly US-customers. Regarding taxes, a lot have actually the same case as yours - they weren't necessarily from Japan but they were able to start operations without having to worry about taxes in the country they're selling to. Perhaps it'd be best to ask the authorities in Japan if you'd need to set-up taxes in the country or in the US. I'll get back to you as well regarding this as I also asked someone from the Spocket team who's knowledgeable about this.
In the meantime, have you already decided on what dropshipping app to use to source out products from? Since you're planning on selling to mainly US customers, perhaps Spocket is an app that you can use. The platform has loads of US suppliers who can ship to US customers in as fast as 1-3 days. Plus, the platform offers a wide range of quality product selections that might interest you. Should you be interested to know more, you can simply head on over to Spocket's official website to learn more or join Spocket's official Facebook group. As well, you can respond to me here for questions you may have about the platform.