But if one checks the class availability (Z9, etc...) and determines which flights are relatively open, the SWU (and not just SWU, mileage upgrades as well), must have a very good shot, no?

As premier exec in the past, I've usually (80%+) gotten the upgrades with miles on the international segments I've flown, so I am guessing with similar travel patterns 1k must be at least 90%+ chance