You might find it best to stick with UA. If you fly in cheap coach, you’ll find it difficult to get to platinum status on ANA (where the useful frequent flyer benefits start) because discounted fares only earn fractional EQMs. Also, ANA miles expire three years after you earn them, so it’s hard to amass a lot of them.

I’d apply for some US credit cards before you move, and collect as many sign-up bonuses as possible... you will not find American-style credit card bonuses in Japan.