Investing Cash in Japan

I'm working here as a foreigner in Japan, and I am looking for investment advice.

The only slight difficulty however is that I get paid fully in cash, and while all of my taxes are done by my company (I'm not asking for money laundering advice), I think there's a pretty high chance they are not paying in full.

Thus, I'm currently looking for a way to start investing my cash, or atleast diversify my holdings, since it's currently just sitting in envelopes.

I will have around ¥1,000,000 per month I would like to spend, plus nearly ¥3,000,000 at the moment.

Thank you for any advice 🙏

Edit:

I get paid in cash and then give a cut to my company which includes them doing my taxes. I am also on a student visa, so I'm not really expected to report my income.

My company has told me they are very confident in their lawyers. Additionally, since I am paying them enough for tax anyways, I have been told I will very unlikely be held liable if anything does fall through. I have been advised not too declare too much money in Japan, so this what I'll do.

I'm just curious if people have any advice for investments I can make with just cash: purchasing gold, art, vintage stuff ….
As I am completely unfamiliar.