NISA through IBKR tax implications

I’m a US citizen living in Japan. I’m considering opening a NISA account through IBKR, but I’m not sure if it’s worth the effort.

My understanding is:

– Japan taxes capital gains at 20.3%

– US taxes long-term capital gains at 15% for my income level

– Without NISA, I’d pay Japan tax and use the foreign tax credit to offset US tax

– With NISA, Japan tax is 0%, but I lose the foreign tax credit and just pay 15% to the US

So the net benefit of using NISA is basically 5%

Is that correct?

For anyone doing this, are there any other taxes, limitations, or fees I’m missing that would make this not worth it?