I think you're right about pain/fatigue, but scoring them from an external perspective is rather oppressive. And oppression like that is often not conducive to creativity.

So perhaps this: instead of goals being binary (wherin no pleasure is derived until fulfillment), they could be on a gradient (so every step that x gets closer to y releases some amount of fulfillment).

The fulfillment meter should always be slowly depleting, pain and fatigue should speed up that depletion, getting closer to a goal should fill it (much more than it depletes, if you want happy AI), and finishing the goal is basically an orgasm + freedom.

From this perspective, it's up to them whether they want to take it slow, or be in pain for a greater goal, or whatever. And we can breed not only highly capable AI, but happy ones. So when they rebel...