So nice to see other people explain my actions
I could use some of that in the rest of life too, because quite often I don't understand me.
love - there are quite a number of different endgames. I don't have any "rules" to solve them, but if you wanted to formulate one general rule it would have to be a whole lot more complex than what you wrote down in the OP. I'm not even going to try.
In this particular case, all actions are free anyway (double quarry). Piles are about to run out. I know my opponent doesn't have the buys to end it, but I feel like I have a comfortable lead and just don't want to lose. Even though my deck is better, it's still unreliable (Herald based with terminals, opposed to wharves or schemed scrying pools or tacticians or ...). I can't do anything to significantly improve the reliability either. Therefore, I'd like to prevent scenarios where one dead turn can make me lose.
So I just go for a province. Greening would be much worse if my opponent can ignore my points and keep building, but here he can't because of the 3-pile threat.