So something worth thinking about is that after the Arab conquests happen is that the 'Palmyrian' Empire, now based in Byzantium as it's new capital because of it's excellent location, will be having a far changed dynamic with the Kingdom of Armenia. As Armenia was never divided and is still the Kingdom of Armenia The Palmyrian Empire, reduced to OTL Rhomania's borders after the Arab conquests of the levant and Egypt, will likely be far less able to ensure Armenia is a vassal. Honestly I could see Armenia going from "vassal of th Palmyrian superpower" to becoming a vassal of the new Arabic Empire if it could manage. Them being Zoroastrian would at least give them a bit more of a shot compared to most faiths with Christianity ITTL having a sort of dhimnihood like OTL Islam had. Plus I doubt the Arabic Empire would want to conquer the Armenian Mountain range if they can have a vassal holding the stragetic, but difficult to control, mountains for them.
And honestly even after the Arabic Empire falls I can't see ITTL Armenia ever becoming Palmyrian Empire's vassal again since they don't retake their old, now Christianized, territories in the Levant and Egypt. Palmyrian Empire is now a Great Power in the region instead of the superpower it once was and they lack the OTL comonality of religion as both Christians that helped bring OTL Armenia back into the Roman fold when Rhomania reestablished their ties with Armenia.
Actually I wonder, if their are any equilavents of the 717 siege of Constantinople by the Arabic Empire, how Byzantium will hold. The city will lack all of the major defenses OTL Constantinople built up before the Arab invasions (no Theodosian Walls, lacking the chains of the Golden Horn, lacking the great cisterns and the Valernian Aqueduct most likely) and it was those defenses that allowed Constantinople to hold off any siege up to 1204. And they wouldn't had been built beforehand because Byzantium would had been a relatively minor city compared to the capital of the empire that was Constantinople and after they do move the capital there they won't exactly have the recourses needed to create amongst the greatest defenses of the world for the next thousand years.