I write:
But this is not necessarily so. It states "Son of David, king in
Jerusalem" (Qoh 1.1 NRSV). All of the kings of Judah bore the
appellation "Son of David" just as all the rulers in Damascus bore the
appellation "Son of Hadad" and various Aramaean dynasts bore the
appellation "Son of (PN)" as noted in Assyrian documents. In view of
this, it could be attributed to any of the rulers of Judah, including
Jehoiachin who died in Babylon in the late sixth century.
For this practice, I recommend the following (I'll add other citations later):
Hélène Sader’s, _Les États Araméens de Syrie: Depuis leur fondation
jusqu’à leur transformation en provinces assyriennes_. Beiruter Texte
und Studien 36. Wiesbaden: Franz Steiner Verlag, 1987.
Qohelet neither states nor implies that Solomon is its author. It is
*traditional* to assign the book to Solomon through an intertextual
reading of Ecclesiastes in light of the MT text of 1 Kings, in which a
picture emerges of a repentant Solomon late in life providing sage
advice to others so that they will avoid his own various earlier
mistakes. But he was not the only king of Judah with wealth,
concubines, and so on, to which such reminiscences would apply. But
it could just as easily, and more fittingly in a linguistic sense, be
applied to the former king Jehoiachin, revisiting his own prideful
state as a king of however brief duration, and his direct suffering of
the consequences of his actions and decisions, placed in context over
the course of a difficult life, which lasted until at least about 560.
I suggest this as a distinct possibility.