Do I need to be a Sole Proprietor to file Income Tax Return for Side Income?

Hello, I am new to freelancing here and have a few questions about income tax.

I am a 正社員 at a Japanese company on a normal work visa and recently I got a freelance job outside of Japan that would pay me about 120,000 yen/ month.

I would like to know if I need to become a Sole Proprietor to file for the income tax return to NTA.

If so, I am unsure about how to deal with the health insurance and pension since I am still employed in my current company and they are taking care of it.

Are there any other legalities that I should be aware of other than notifying the government that I will engage in activities outside of my work permit?

I work as data analyst in my current company and I will work as an analytic consultant for the side job

by BlockMaster42