If that's the case, it could be that a "substantial move" indicative of this migrant opportunity-seeking mentality doesn't require immigration or the crossing of nation-state boundaries at all. I wonder if there's another way to phrase this to include someone uprooting themselves from the American east coast and moving to California, for example.

Similarly there are parts of the world where crossing a border to live is nearly as easy (e.g. within the EU). What's the barrier you're trying to measure? Leaving familiar people/surroundings?

[Edit - added EU example and question]