Were the Lakhmids loyal vassals of the Sassanids at this point in time? Perhaps Shapur might grant fiefs to their noble houses in order to dilute the influence of the old Arsacid nobles, at least as far as providing wartime levies. If not, then at least with his new wealth Shapur could afford to hire them as a standing force of light cavalry to protect trade caravans and help put down rebellions.
How does the conquest of the Levant and Egypt affect the political situation in southern Arabia? Apparently in 260 AD only Hadhramaut and Himyar were left to contest control of the region. IOTL Himyar conquered Hadhramaut in 300 AD, but with the Sassanids now ascendant from the Red Sea to the Persian gulf, does either nation have an advantage in getting them to intervene in the conflict? How does a Persian monopoly on the inflow of frankincense and myrrh to the Mediterranean affect the profitability of that trade for southern Arabia?
It would be approximately 65 years before the Lakhmids would be subjugated by the Sassanids. At the time of 260-270, the Lakhmids are currently moving between the lower Nejd and Ahsa regions of Arabia and thus not in their historical range of al-Hira (near modern Kufa), the lower Euphrates and the coast of Bahrayn-Kuwayt. By around the late 290s, we might say that the Lakhmids arrived near the Euphrates and by at least 310, had constructed al-Hira and began their much wider disputes with the Sassanids. In the 270s, it is known that the Lakhmids did attack Sassanid lands in Iran, but this was through piracy along the coastal regions of the Ahsa and Oman and were small-scale. It is only whence the Lakhmids encroached upon Sassanid Babylonia, that the Sassanids turn their attention southward. It would be until Shapur II (309-379) that the Lakhmids would be dealt with by the Sassanids and made a vassal around 334-335.
The Lakhmids have no noble houses with which to bestow these lands to, or at least none who would be recognized as such by the Sassanid imperial complex. In the 4th and 3rd century, the Sassanid policy have not suffered any defeat by the Arabs and will not see any benefit to giving what they may have seen as an inferior people any land. Their duty was to submit and pay tribute, not to be given land grants. Further, assuming the Sassanids did gift land to Arab tribes across Syria, what sort of troops would they receive from this? The Partho-Dahae were particularly valuable as they were the only group outside of the steppe region providing a large assortment of heavy cavalry, horse archers and additionally a large assortment of other troops derived either from their subjects or from mercenary and slaves that they owned. The houses were also not simply feudal land owning classes likened to Medieval Europe, these were in essence independent kingdoms who accepted Sassanian imperial titles for the sake of maintaining a certain decentralized status quo. Sassanian legitimacy thus, oddly revolved around their acceptance of total noble control east of the Tigris and upon the houses agreement to their rule. Sassanian personal lands in Iraq and the various free cities (which were either Babylonian-Assyrian-Aramaic-etc or were old Hellenic free cities used by the Seleucids) did not provide the necessary military resources that made the Sassanian armies famous to Rome, that being horsemanship, especially horse archers and heavily armored Saravan-Cataphractoi. It should also be noted, that only a minority of the free cities constructed during the 424 years of the empire, were constructed by the Sassanian clan, the majority were constructed by the great houses and thus these cities would have been subservient to the noble patrons; this situation is partly the scenario for later Sassanian royal jealousy against the noble houses of Karen, Ispahbudhan and so forth (particularly during the reign of Khosrow I and Hormozd IV).
Regardless, I find it more likely for the Sassanian royalty to gift some lands to its nobles, especially the Ispahbudhan and possibly the Surens, both of whom held the most power at this time in the imperial court. The Ispahbudhan having the privilege of marrying their daughters to Sassanian monarchs and or arranging the marriage and the Surens who were the Parthian house Sistan by proxy the entirety of what was formerly the southern reaches of the Kushan empire. In fact, it is these two families that likely solidified the Sassanian royalty after the battle of Hormozdgan and the death of emperor Artobanus IV (the last Arsacied emperor) and gained from the other noble houses, acceptance of the Sassanian rule and formulated the confederation of the later Persian empire. Then after distributing some lands near the noble houses, it would be the best course and most likely course for the Sassanian royalty to utilize the local populace as free cities and areas ruled directly by the Sassanian kings as personal fiefs. For this, the Sassanians will need to create governor positions and military generalships, create naval sectors and so forth. In otl, the Sassanid royalty did not require as large an apparatus for governors and so forth, as the only areas under direct Sassanid control, were some areas within the Ahvaz, Fars and then their capital region of Babylonia. The rest of the empire was ruled directly by noble houses who collected taxes in their own name and then sent the portion necessary to Cteshipon as a due. Until Khosrow I, there was no Sassanian tax collection in Iran and areas east of the Zagros or north of Ninevah, this is likely a long set precedence by 260, which had been set for about 500 years, since the fall of the Seleucid empire, that the rebellious and freedom seeking Dahae-Pahlavi clans never paid taxes, rather they collected taxes and paid a due.
Thus, the addition of these lands in the west, used to a bureaucracy and knowledge of this sort of system, will be likened to a treasure trove for the Sassanians. Sassanid monarchs may seek alliances with the local peoples, especially courting the Jewish populace and use their know-how to create a bureaucracy of some sorts, to collect rudiment taxes and collect tolls and so forth. Additional income through this, will allow the Sassanids to funnel more resources and gifts upon the noble houses. During the majority of Sassanian history, while the monarchy received dues form the noble houses, the nobles also received supplements. These supplements acted as almost refunds on portions of the dues that the nobles gave to the monarch. Thus, at any time, the nobles would present a report of some kind to the Sassanian monarch, this report would include mainly requests. As far as substance, requests usually amounted to a request for a certain amount of armor or a certain amount of resources to be able to maintain a particular unit and the Sassanid monarch was essentially bound to accept these requests and refusals often led to a deposed monarch. All of the so-called good emperors prior to Khosrow I, were the ones that essentially followed the customs regarding the nobles and their requests, which likely, as we say, derive from 500-700 years of blissful decentralized rule under the Arsacids, Kushans, Bactians and Seleucids. Part of the issue in later eras, is that Sassanian treasuries began to become less abundant as time moved on and thus the noble requests became more and more and more of an hindrance. Especially considering a reality wherein the nobility became known for cutting corners and requesting items they already had or listing infantry as cavalry so as to increase their budgets. It also was an issue when certain noble houses, such as the Surens cut corners and used their budgets for hiring mercenary to send to fight wars, instead of training cavalry (it was far cheaper to higher steppe horsemen from the Huns, Kushan, Turks, etc than it was to train a hardened noble cavalry man). However, this was an issue approximately 300 years after the POD, thus the Sassanids even without the western lands have enough to pay the nobles a refund; thus this additional income; is used for the sake of building larger warchests fro hiring mercenary, creating a standing army for the west (leave the east to be protected by those who know the region, the nobles), building more free cities, expanding and creating a navy, lavish gifts to the nobles, a new campaign into Anatolia, etc etc etc...
Regarding trade policy, the Sassanian system was even more decentralized than their tax system. Namely, the Sassanids permitted the nobles to control all trade in their lands, which constituted some 80% of the empire.This means that the nobility had their own tolls, their own regulations and their own systems. The nobles thus controlled the wealth of the Sassanian capitol region of Babylonia, at least by land. This included noble control over the important trade of silk, gold (from the lands of the Saka), spices, and all goods peddled across the silk road; not to mention one of the most important, the resource of horses, which the Sassanian fiefs had very little possession of, as Iraq is not wealthy in the equine breeds (one of the main interests that the nobles had, was the acquisition of horses from the steppe, to be used in their units and to maintain a semblance of their steppe origins). Another issue Sassanian royals had, as early as 270, was that unlike the Seleucids and Bactrians, which gathered much strength through the slave trade between free cities (the Seleucids created the concept of the free city in the Eranshahr, one of the greatest legacies of the Seleucid empire, is their creation of a tradition of city creation for monarchs and designating certain cities as free from typical restrictions), the Sassanians could not gain from this, as for reasons not clear, the slave trade in the east had declined enormously compared to prior centuries during Hellenism or the Saka-Scythian ruled periods. This lack of slave economies centred upon free cities benefited the noble houses, by lowering the number of merchants with slave labor, making the nobles and their subjects far more valuable and powerful. With that in mind, the Sassanid acquisition of the western regions will be massive boon toward trade income, and will allow more resources to flow throughout the empire than previously. As long as Sassanian interests do not move toward controlling the noble's lands, I cannot see them protesting or becoming agitated. The main issue here, is that the Sassanids are not well versed in the task of controlling these sea trade routes. The best idea is for the Sassanids to utilize the existing populace and allow them freedom to manage this system, perhaps simply implementing a due system like with the nobles, but without any sort of refund system like with the nobles military requests.
In terms of raising troops, my advice would be to not recruit form these populaces much more, and especially not rely on them for campaigns. The Sassanid prowess exists in the noble houses and their armies, not in raising a polyglot foreign army. At least, do not rely on these. It would be better to have a standing Persian army, than an army made up of these newly acquired subjects.