What is a word or phrase for "The point in time when the work pays off and it takes care of itself from here"
I would go with critical point.
"Critical point is a wide term used in many branches of mathematics. When dealing with functions of a real variable, a critical point is a point in the domain of the function where the function is either not differentiable or the derivative is equal to zero."
Where the given point is at either the top of a peak or hill in the graph, or, at the bottom of a low or valley in the graph. Or, at a saddle point, where the graph changes from being a valley to being a hill - at a given point somewhere in between.
In applied mathematics, critical point refers to stuff like the point at which the greenhouse effect becomes irreversible.
In nuclear physics, critical mass is about stuff like how much mass is required to sustain a nuclear reaction.
Another option is tipping point.
"The tipping point is the critical point in an evolving situation that leads to a new and irreversible development. The term is said to have originated in the field of epidemiology when an infectious disease reaches a point beyond any local ability to control it from spreading more widely."
There is also turning point, point of no return, and, I guess, so on. Breaking point. Point of inflection.
- It's all downhill from here.
All downhill from here
- Increasingly easy (as if literally traveling downhill, as opposed to uphill).
- I'm sure I can finish the rest of the experiment on my own – it's all downhill from here.
[Farlex Dictionary of Idioms]
But note that the 'downhill is a bad direction' metaphor shows in an equally valid contradictory meaning:
- Steadily worsening.
- Once my parents open my report card and see how bad my grades are, it's all downhill from here. [op cit]
.............
An unambiguous, but informal, alternative is home free:
home free [Informal]
having finished the worst or most difficult part of something
- If the refugees can make it as far as the United States, are they home free?
[Macmillan]