Hal 9000 said:
I’m hoping to avoid mentioning either a “thousand years” OR a “thousand generations,” because of the ambiguity between the OT and PT’s depiction of the timeline, EU material notwithstanding.
Just don’t mention it, there is no need. That’s why I would change “menace” by “ancient menace”, that way you get some kind of a “long time” idea without using “years” or “generations”.