Unfortunately, although the threat that Syria and Remaliah will be destroyed, that threat will pale before the reason those two will be destroyed. Jehovah will bring upon Judah the king of Assyria.
In verse 18 we have the fly from Egypt and the bee from Assyria. Both Egypt and Assyria are enjoined in war, but Egypt is the lesser problem. The mightier, and more destructive, will be Assyria.
In verse 20 Isaiah discusses shaving with a razor that is hired. There are two parts to the image. The first is the part about being hired. That means that Assyria comes as an agent of Jehovah’s wrath. They will serve his purposes, hence they are as though they are hired for that task.
The shaving refers to an Assyrian practice of shaving captives. It is suggested that this might have two reasons. Perhaps one is hygienic, but a stronger reason is the second. For a culture where the male beard is associated with virility, shaving the beard is an humiliation. That humiliation fits the context of the prophecies of the destruction at Assyrian hands.