As you know, in tone and content Ecclesiastes is very different from the other books of the Old Testament. But I do detect in Ecclesiastes a shared concern and goal. Ecclesiastes just gets there a very different way.
Specifically, I think idolatry is the overarching moral concern of the Old Testament. Idolatry is what the Torah is concerned about, what the historical books are concerned about, and what the prophets are concerned about. Even in the Wisdom literature, Psalms is also concerned about idolatry.
And so is Ecclesiastes, but with a key difference. For most of the Old Testament the issue of idolatry is framed as one of love, loyalty and fidelity. Idolatry is unfaithfulness.
But that's not the approach in Ecclesiastes. In Ecclesiastes idolatry is approached through the predicament of death. In Ecclesiastes, idols are not condemned for being instances of infidelity, they are dismissed as being vain and meaningless in the face of death. Since nothing lasts, chasing these idols is like chasing the wind.
In all this there's a deep sympathy between Ecclesiastes and the rest of the Old Testament. The differences we note in Ecclesiastes has more to do with its radically different approach to throwing cold water on the idols we pursue.
In calling us away from idols, Ecclesiastes uses death rather than the wrath of a jealous God to bring us to our senses.
