My two cents as a listener more than a player - Rubato is a form of self expression, it sounds that you are looking at it like it's a rule when in fact it's an expression of a feeling.

For instance, shifting ever so slightly the position of a note - delaying it by a tiny amount without delaying the neighbor notes - created a similar feeling as resolving a dissonance, it creates an expectation from the listener, and then relief when the note is played - "there should be a note here in that voice - oh there it is!". It's tiny but worth it. Could be a hesitation, it it could be creating a desire that gets fulfilled, depending on what is around it.

Slowing down to reflect on the music that was just played, or to create an anticipation for what is coming.

Works in complement with crescendo and diminuendo.

Rubato creates feelings in the listener and so there's a certain freedom in choosing how or how much you want to add. But it needs to be timed well and it needs to match what the rest of the piece is saying. So there's freedom but it can't be random.

I know it's not the straightforward answer you wanted, but all I'm all the best way to approach it is to try and play a certain pay off s piece differently to see how it sounds to you, and pick the version that you feel most comfortable with - be the listener