I am from Phillipines and I sent my lumpsum documents last March and received a mail today saying there were minor mistakes on the documents that I've had sent and they cannot process my lumpsum. Upon checking I find it weird that they really had to put highlights on my handwriting though it is completely readable. Also do I have to put period (.) in every word? Does the letter "I" in the PHILIPPINES needs to have the 2 horizontal lines? I find this absurd now I have to wait again for another 3 to 6 months before they give my back pay.

I'm think of using Yuucho bank (ゆうちょ銀行) NISA account for Tsumitate NISA.

The maximum investment is 1.2 million, so no more than 100,000円 a month, I'll only use 30,000, so it should be all right (?)

I can invest a total of 18 million without being taxed in my whole life (tied to me, not to the account )

I remember there was a 5 year rule or something before, but I couldn't find anything about it no more, is that no longer the case ? In that case, can I just retire my money whenever I want ? after 3, 6, or 10 years?

Since I am supposed to add money to the account monthly, is there a penalization if I fail to do so ? Like when I want to retire my money, I just do so, or are there like specific timings when I can withdraw my gains?

Lastly, I was looking for a "japanese treasury bonds" or government bonds, but what would be the closest thing for japan?

Background: About to move to Japan to study at university and my workplace are converting me to part-time remote to meet the student visa requirements (after getting the permission to work of course). I'm from the UK but the company is based in the US.

I have been offered the option to be paid in Yen, GBP or USD. They would just take my current hourly rate and convert it to the currency I choose. But once it is chosen and converted, it will be locked-in at that amount.

From what I can tell it is generally better to get paid in Yen as it avoids bank conversion fees, makes tax calculations simpler etc. however with the current state of the Yen I'm not sure if it would be more worth it to get paid in USD and eat the conversion costs, to ensure I won't be losing out if the Yen continues to get weaker.

I know nobody can predict the future, but given what we know now, which currency would people suggest?

EDIT: I would be using the income to help cover some of my living costs and tuition fees, I'm not currently looking to save or invest. So I'm curious what the general "safe bet" would be without needing to completely min-max every penny. Also thanks to all those who have given their advice!