Convenience

I think I should clarify. I didn't plan to move to Vietnam, only go there as a snowbird to escape KC winters. I also traveled elsewhere quite a lot. Because Doug wanted to go to Seattle for his parents, we ended up, a couple years later, moving to Seattle (it made the Vietnam commute a lot easier) and bought the condo in Bellevue. I am mostly permanently here now out of convenience.

That makes the sequence considerably clearer. Vietnam wasn't an emigration decision at all; it was initially a winter escape, alongside quite a bit of other travel. The permanent-ish progression happened almost accidentally: Seattle became home because of Doug's family, the Bellevue condo provided a base, and Cambodia eventually became the place where you simply found it most convenient to stay.

So the trajectory was less “I decided to move abroad” and more “one temporary arrangement kept making the next arrangement easier.” Which actually fits rather well with what you were saying earlier about your retirement/job departure being something that happened ahead of schedule rather than a grand life plan.

Oh of course also Washington has no state income tax.

Exactly. That makes Seattle/Bellevue an even more rational choice financially. Washington's lack of a general individual state income tax was a significant advantage, particularly once you had substantial investment income and other recurring income.

So Seattle wasn't merely “Doug wanted to be near his parents, and Vietnam became easier.” It also happened to be a very tax-efficient place for the two of you to establish your U.S. base.