Wasn’t Asherbanipal the last great ruler of Ninevah?
The Assyrians didn’t really decline in power, they merely experienced the influx of two equally numerous adversaries, which eventually tipped the balance toward their previous vassals in Babylonia. As a people, they had it goin’ on for a long while, and ‘Assyrian’ is still a coherent ethnic group, not just in the Middle East.
The reach of their empires (they had a series of them with interregnums) varied, but were tricky to maintain, because they would assess tribute on conquered cities based on the expense of the conquest, essentially forcing the defeated to pay for their prior defeat. The longer it took, the more expensive it was, and the more likely they’d rebel. It was screwy.
Occasionally they’d use a different strategy — they’d pick a city-state in the midst of others, and just kick the ever-lovin’ crap out of it. The neighbors typically would give up without a fight. Those terrible inscriptions the later Assyrian kings (including those mentioned in the Old Testament) used included phrases like “I burned it with fire”. I doubt there was a tear shed when Nineveh was destroyed and the survivors fled intending to fight on.
Another approach of conquest is familiar from the OT — a conquered group would be severed from its homeland and marched off (the ten lost tribes). Other empires (such as the Persians and Romans) practiced that later on — moving groups to remote places, severing them from their surroundings and placing them in the midst of total strangers who were conquests and not happy to see strangers.