Retirement Strategy/moving countries

I(US citizen) have been doing research on retirement investment stat on behalf of my Japanese fiance and was hoping for some advice before return to the US this month.

My fiance (Japanese citizen, 32, Japan resident) will be able to start saving about 275,000 yen/month. We plan to start K1 visa process soon and expect he will be able to move to the US in about two years and eventually get a green card. Long term we may retire in Japan but aren’t certain.

He is the process of beefing up his emergency fund but we werent sure if he should start investing in US domiciled EFTs through IBSJ through taxable accounts or open a NISA account (looks like he can’t keep nisa if he isnt a Japan resident?). We have pretty much eliminated iDeCo as an option. Our other choices is him just holding cash and investing when he’s in the US.

I am not sure if I’m overly concerned about his future tax status at this time. Appreciate any guidance!

by tired-tied