For me its a lot simpler... a typical Hebrew parallel. Check the repeated concepts on day 1 and 4, 2 and 5, 3 and 6, having the 7th stand alone for emphasis... being the type of the Sabbath. The day, specifically, the evening and the morning are the opening and closing parenthesis of the literary parallel. A prefigure of six days shalt thy labor and do all thy work...etc.
We moderns may have read far more literalness into the account than was perhaps originally intended anciently.
Interesting thought which could still be true in spite of the fact that God carved the 7 day cycle in stone with his own finger when He gave Moses the 4th commandment.
It is intriguing to me that the Sabbath and the creation are so often inextricably linked in the Pentateuch.
Why?
Very good point!