There really is no reason to think that the original author thought "day" meant anything other than "day" here. The text mentions seven days and means seven days.
But I also don't think the intent was a play-by-play in a literalistic sense. In poetry words can retain their direct sense without necessarily being intended in a literal and wooden way.
"My love for you is like a red rose." (I'm stealing this example from N.T. Wright) is a poetic statement, the rose here is just a rose, but the use of simile is understood since the idea of the rose is being used in comparison to the love of the poet for their beloved.
So the days of creation are days, in the plain sense; but being used in a poetical narrative for a purpose beyond the mere "this day, then that day" sense. This is why I think the
Framework Hypothesis makes the most sense of the text.