"Then there are what you could call verse-fillers, repetitions of entire lines in slightly different words to pad out a verse"

Well, if it's different it's not a repetition. For one thing, there's a difference in nuance between rising early in the morning and rising before the break of day. The latter is much more charged, implying an urgency, a cut, a... "break"! I don't call that padding; it's not as if words are merely a vehicle that allowed a singer to sing. (They may be that, but they're always more.)

But even in instances where it really is repetition, again, it's not filler - it's essential. Repetition is an effect; there's nothing superfluous about it. It's poetic, a legacy of ritual. Is a song's chorus padding? Are the repetitions in religious liturgy padding?

I think it's also a stretch to suggest that various stock folk adjectives are simply there on account of convenient meter. If so, why stop there? You might as well say that use of the word "horseman" or "maiden" is filler. Why is "bold" necessarily any more or less significant than "fisherman"?