Not OP, but I live in Asia and am sporadically changing countries. Japan either requires you to have a job offer prior to moving, or show sufficient funds to live there indefinitely (around 2k USD/month; though that won’t get you far in Tokyo for example). Remote work does not qualify, it’s either a domestic job or income from pension or investments.

After 10 years of living there legally you qualify for a residence permit.

The alternative would be through marriage.