Well we do not need a Syriac source for the knowledge that the Khawarij revolt ravaged the Assyrian homelands and required both Kurds and Abbasids to quell. Further we know that governors and millitant guardians of Ninewah were all Arab or Turkish Muslim of the Abbasid period. A Syriac source is not needed.
It would be better to have the sources for a number of reasons
1) Because it may well be that it wasn't the Khawarij revolt is used as a scapegoat for Abbasid and Kurdish atrocities.
2) Because more information is better, especially information that has more familiarity with the local area than other sources
3) Governors of a separate culture to the people have a different perspective - whilst the Arab/Turkish governor may have seen himself as benevolent (and reported as such), the people he rules may have seen a measure of ignorance of their culture as total disregard for their traditions - if we're to be favorable to the governor. In this case in point - the governor of the region may have chronically underfunded the local guard to embezzle money, but certainly none of that would be reported by the governor - but the people may notice - making the revolt more dangerous than it needed to be.
More sources, especially those that can be confirmed, is always better.
Getting back to the OP - another option that I had missed, is the Byzantines can do something similar to what I was recommending - if they can defeat the Persians, training the Assyrians as a demographic to lean on for local stability is possible. Alternatively, if Yarmouk is a Roman victory, with Persia in a state, and the Romans turning back the Caliphate, Persia may well survive in a state of civil war. The Roman army is still exhausted - but the Roman state is still the wealthiest by far - it could also fund an Assyrian state to be allies with, but it would require a charismatic leader to emerge.
Also, all this talk of the "defeated and pathetic Assyrians" is starting to smack of Martial Races theory. Can we not fall into that rabbit hole?