It takes 50k ANA premium points, not miles, to reach Platinum / *A Gold status. Depending on the fare class you book, you might be surprised at how many premium points you can earn, especially on Japan domestic flights. My company books full flex economy fares for my domestic business trips and those earn more points than when I book discount economy to the US during the holidays.

Based on your intended amount of flying, I would think you have a good chance to reach 50k points.

I switched from UA to ANA and believe it's worth it for the service level alone.