First, is the True Patronus really that overpowered?

Considering the protagonist got it simply for being the protagonist, it turns dementors from unstoppable soul-eating monsters to a trivial threat (they can't even see you), and it may be able to stop the (unstoppable) killing curse ... it's pretty damn powerful. Not "winning-every-conflict" powerful, tipping-over-the board powerful.

Here is an alternative interpretation of the prophecy:

THe one: Harry With the power: The True Patronus To vanquish the dark lord: The Dark Lord is not Voldemort, but rather a personification of death (maybe even a literal personification of death such as the grim reaper as a King Of Dementors. approaches, born to those who have thrice defied him. :Heroism is often called 'death defying'. Alternatively, it could be a wierd interpretation with Harry being born from his biological parents to his adoptive parents. His adoptive father is a biochemist (IIRC) who works in a field that is probably connected to medicine. Perhaps he has made three significant medical discoveries? Although I am not sure about the whole thing with being marked as his equal or whatever.

I have to admit, I find these alternaive interpretations ... unlikely.

Hypothetical Apocalypse Spell: A fully general preference for death over life. It's unlikely that anybody would ever cast this. Possibly connected to the ritual that will summon death itself? Ritual to summon death itself: True Patronus may be the counterspell to dismiss death.

I'm guessing that "Apocalypse Spell" is where dementors come from.