I feel like "she'd been saying goodbye too often" is "zbyt często żegnała", but adding "zbyt często już żegnała" is like "she'd been saying goodbye too often, but now she's fed up with saying goodbyes, and doesn't do that anymore".
Of course "Kiedyś żegnała się zbyt często" is more explicit and understandable, but not as efficient for this competition :)
It doesn't really capture the intent of the original sentence either. To convey the same idea you'd have to say something like "Za wiele razy mówiła żegnam"
"Say goodbye" doesn't necessarily mean "to speak the words 'goodbye'". I'm not an English expert, but I think that waving with your hand is also "saying goodbye". And if that's true, then "żegnać" is the same as "saying goodbye".
Of course "Kiedyś żegnała się zbyt często" is more explicit and understandable, but not as efficient for this competition :)