Looking at doing a Sole Proprietorship in Japan. Would I have to join the NHI immediately or could I stay on my spouse’s company insurance until I start making enough income over the 1,300,000 yen limit for spouse’s additional income.

Looking at doing a Sole Proprietorship in Japan. Would I have to join the NHI immediately or could I stay on my spouse's company insurance, which I am currently on, until I start making enough income over the 1,300,000 yen limit for spouse's additional income. Also how does pension work in this situation?

Finally if I do need to or when moving to NHI and Pension how much is it for Sole Proprietors?

Any advice or experience doing this would be helpful.