Marry. File I130 asap. The spouse process will be 18-12 months until the visa is in hand. Your temporary stay in Japan
will not be an issue. You will
need to demonstrate that you will be returning to the USA to live in order to meet your domicile requirements. You must enter the US before or with your immigrant spouse when that time comes. Keep your US state drivers license, voter registration etc current while away.. use a safe reliable address of parents, sibling , friend etc as your US mailing address.
Enjoy the experience !!