One issue that is seemingly being dismissed or forgotten under the western Sassanid front, is the political situation in the east in the next 100-200 years.
In otl, the fall of the Sassanid Empire led to the expansion eastward of the Umayyad forces and the wider dar al-Islam, against various autonomous states, tribes and several rising powers. Over the course of 700-860, these groups were primarily subdued and conquered in the immediate vicinity of the Caliphate and the rest failed to make gains politically. Thus, in the grand scheme, the Islamic influence and thus Abbasid pressure became the strongest, despite the rapid decline of the Abbasid power in the 840s. These three immediate players that the Sassanids will need to deal with are:
1. Gokturks and the various soon to be divided Turkic hordes of many tribes. These in otl, were initially foes on the northern and eastern front with the Abbasid. These groups however were generally divided and unable to resist the Arab forces which preyed upon them in slave raids and various wars of pillage and loot. This policy and pressure surely lowered the power of these Turkic hordes and surely postponed any major Turkic invasion. In otl, the Sassanids already had various relations in the steppe, most especially with their wars with the Hepthalites, whom they defeated with an alliance with the Turkic hordes now propagating across the steppe replacing much of the Iranic steppe hordes previously in vogue since the times of the Assyrians. The Sassanids thus had already dealt with the encroachment of the steppe and dealt with these foes in ways not dissimilar to the Byzantines, that is, alliances with steppe nomads just beside the encroaching horde. This is perhaps the best way the Sassanids can deal with these hordes, whilst invading the steppe with a modestly powerful army to pillage and force the warring nomads further north.
2. Western hordes. These would be those Turkic or otherwise nomadic hordes that would exist in the Pontic Steppe or nearby east. Thus, this would include the powerful Khazar units which began forming into cohesive powers near the end of the Sassanid period and the more fearsome Pechenegs who arrived shortly after the Khazar. In otl, the Umayyad fought extremely costly wars with these Khazar. Khazar entities invaded and broke through the Umayyad defense in the Caucasian mountains and pushed into Iraq, shaming the Caliph and creating a major detriment to the Umayyad morale. Further, the Umayyad invasion of Khazar territory, shows a tale of difficulty, with the Khazar fleeing northward before the Umayyad army's counter invasion. The importance of the Sassanid to maintain this border is important and possibly an alliance with said Khazar is possible.
3. Tibet: The rise of the Tibetan empire is not something that I am particularly versed in, however, the Tibetan empire int he early days of the Umayyad, exerted large influence in the region to make its own gains and counter those made by Islam. Especially Afghanistan, where the Tibetans countered the Umayyad conquest by invading and executing the Umayyad emir; this was soon pushed back by the Umayyad who returned and invaded the Kashmir region. Later, the Tibetans and Abbasid would form what seems to be a pragmatic agreement against the expansionist Tang. In a tl without Islam, the Sassanid will likely be the second largest player in this roulette of the east, between Tibet, Tang and the various Nomadic Hordes, as the Caliphate was and was the eventual victor in this geopolitical struggle.
One interesting scenario I thought of, regarding this eastern maneuvering, is a more robust Tang-Sassanid alliance. Essentially, the Sassanid lessen their posturing in the west, to focus in on the rapidly changing systems in the east; especially in regards to culling the Nomadic threats to the north, by assimilation, extermination and pushing west or northward the threats from the nomads. This is then in conjugation with a Tang push westward to secure the 'Silk Road.' With careful invasions and timing, the two could meet very nearly as bordering. The system then works as the Sassanid roam and protect much of the region from nomadic threats and thus the silk road becomes far more robust than in otl, where it ceased to truly exist. How long this lasts however, is beyond me.