Roughly 4 million yen available for investing. NISA lump-sum or optimize for points & taxable fund?

Hi, in the last years I preferred to invest the full yearly 3.6mil NISA threshold in January and over the year accumulate the savings for next year's investment. I realize I am probably leaving some gains on the table by not investing the savings into a taxable account, but in my mind, the earlier I can get the NISA maxed out, the earlier my tax-free gains would be in the market.

For 2026 however, I am not so sure anymore – I have accumulated ~4 mil for January and could invest 3.6 mil immediately into NISA S&P or All Country, but wanted to check if there is a more optimized way to use that money?

One option that I came up with is:

  1. Lump-sum 1.8 Mil into NISA in January
  2. Maximize rakuten points by doing the monthly tsumitate+growth of 150k (1.8 mil yen) – 9000 points @ 0.5%.

This means I would have invested 1.95 mil in January, leaving me with ~2 mil left -> invest in the taxable S&P or All country.

I know a lot comes to risk aversion preference, and I was generally ok in the past with just putting the whole lump-sum in January. I wouldn't be against reducing the risk a bit and averaging a bit more over the year. But I believe I am leaving a lot on the table by not using the accumulated savings that I have by the end of the year.

Any thoughts?

by Iekei_ramen