Did some more research and found EVA's upgradeable ECO Saver economy fares to be cheap (less than $1500) and somewhat flexible. Seems to book into B fare all the time, which has fee-free changes. However, maximum stay is only 3 months, which may be problematic if using home as the destination.

Looked into Japan Airlines too. W and Y economy fares have fee-free changes, but are pretty expensive themselves. And SPG is their only US transfer partner.

Seems like KE is indeed the way to go.