Hi and thanks for your pictures and question about your privet tree and hedge. Since privet is considered invasive in Oregon, I'm not sure why you want to keep it. Since a systemic fungicide did not alter the leaf drop, fungus is probably not the problem.

Whiteflies and aphids do attack privet and may be seen on the undersides of the yellowing leaves if you look closely. If you do see signs of insects, a strong water spray up under the leaves will dislodge them and the predatory insects will help keep them down.

And please don't let those lovely flowers go to seed, as the birds will certainly spread them far and wide.