@gingermarie Im from Eastern Europe so I never had to think for a second if I could find a better career in Tokyo or Back-home
so our views probably vary, but.
15M is enough to live a good and extravagant life in Japan.
Keep in mind that outside of FAANG people won't pay an engineer easily more than that so you will have to move to managerial roles or stay in FAANG.
You won't "ruin" you career by moving here, but further it, even if you decide to go back to US later.
Fresh start is also a huge thing for a person your age, which a lot of people seem to not think about.
A lot of us had made it without looking like a local, having the headaches of getting rejected from houses, struggle with the language etc.
In your case you thats not a problem.
About the Yen, we never know what lies in the future.
Financially and life-wise, I say do it.
Now that you have the energy and chances, fill your life with experiences. You can always get back to the place you are, you have a strong resume too.