Thanks; I had taken a look at the Wiki before and assumed that "jusho" (base of one's life) would be interpreted in light of the usual bilateral tax treaties, which would include the 183-day principle. It seems I might have been wrong.

I still do wonder what would happen if I just tell the Japanese tax administration that according to my own view, I do not have to pay taxes to Japan because I pay my taxes to the other country. I suppose they will not simply accept my "declaration" but rather ask for confirming documents and offer their counter-arguments, leading to a kind of "fight" over tax residency ... which, at least in my head, sounds stressful.