Half-n-Half, in my opinion, your plan is not that unrealistic. Maybe many or most foreigners get hired for their English skills. But professionals who are very skilled in their field, whatever it is, have good chances of finding a job in it here in Japan, too. I know many examples, one of them is a researcher in physics. For taking this kind of path for getting into a company in Japan, you should develop much better skills than average Japanese have. Also you should use these 8 years to built up relationships with possible employers in Japan. Not easy to do, but not impossible. For example, you could go to a Japanese university or some other institution as a research student for a year while you are enrolled in the US. A great way to establishing yourself in Japan after you finished your PhD studies, would be to come here with a sponsor or scholarship. Anyway, you should continue to study what you are most interested in. That's the best condition of becoming successful in it and increasing your job chances.