It's important to note that the Seljuks and other Turkic dynasties didn't just "migrate" to Central Asia. Turkic, Tatar/Mongol, Caucasian, and other warriors served as part of the Slave Aristocracy throughout the Islamic Middle East (at this point, most often called "ghilman," but "mamluks" and "janissaries" were comparable roles). As Slave-Aristocrats, they came to wield great influence over the political institutions of Persia and the Levant, and eventually became landholders within the empires they supposedly served, which they then conquered. The Turkic conquests of the Islamic heartland sort of parallel the fall of the Western Roman Empire -- though popular historiography talks a lot about "barbarians at the gates," one mustn't forget the role of "barbarians inside the gates."
There are 2 types of Turkics who entered the Islamic world:
Professional slave warriors. Or Nomadic pastoralists.
The Ghaznavids were the former and the Seljuks were the latter.
The Ghaznavids were seen as very cultured and highly civilized and the Shahnameh itself, foundation of Persian literature was made in their courts.
As for the seljuks, they were seen as semi Muslim
(even if the Seljuk ruling class accepted Islam, they had very little idea of its actual rulings.... Let alone the Oghuz masses) barbarians who ravaged a great deal of Persia. And pushed for a reduction of agriculture in favour of pastures for their herds.
Completely unlike the Samanids, Buyids, Saffarids, Ghaznavids etc who were all sedentary with Abbasid style high culture courts.
The reason the Abbasids, etc contracted Turkic slave-aristocrats was to curtail the power of their vassal aristocrats. In the 800s, at least, the Turks were outside the internal politics of Baghdad and Samarra -- they were loyal to gold (and Allah), unlike the Caliph's provincial vassals (the Hamdanids, Barmakids, Tahirids, etc) who all had interests of their own. Over time, as it got harder to pay them, the Abbasids gave them land, titles, and political privileges instead -- creating a new class of vassal aristocrats. By then, however, the Turkic ruling classes had been working with the Abbasids for some time, adopting Islam as well as the Arabic/Persian languages. The great universities of Khorasan -- Bukhara, Samarkand, Nishapur, Kabul, etc -- were not just important because of the revival of Persian scholarship, but also because these were the madrassahs which educated and evangelised to the Turks (with the sons of warriors filling other roles, such as scholars, clerics, and viziers).
The main reason the Abbasids chose slave sis because they had alienated all other support bases.
Starting with the Shia shortly after the revolution by betraying the Alids.
Then during the Amin-Mamun war, Amin represented Arab armies, while Mamun represented Khurasani. Resulting in the Khurasani winning and the Arabs of Iraq and Syria being sidelined (which would eventually lead to the revival of Bedouin raids and the Qaramita)
But after Mamun's victory he decided to try ruling from Marw in Tukrmenistan, further alienating the Iraqis.
Then after 6 years he decided to go back, after all Iraq and everything west was in complete chaos, with an anti Caliph being declared briefly in Baghdad. En route to Iraq he declared Ali Ridha as his successor, alienating many of the Abbasid family and Sunnis as a whole. But then he backtracked and killed Ali Ridha, alienating the Shia.
Finally, when he got to Iraq he for some reason gave Abdullah ibn Tahir a semi independent Hereditary governorship of Khurasan. His own powerbase...... Placing his powerbase in the hands of another family.....
The problem was that Abdullah ibn Tahir didn't obey Mamun in defeating the Zoroastrian revivalist Babak Khorramdin. But Mamun had no army to force Ibn Tahir to do it, and didn't have an army himself to fight Babak.....
So he turned to his brother, alMutasim who was building up a personal slave army. He was personally an excellent fighter, able to command the respect of his slave troops, and thus defeated Babak. Thereon Mamun heavily relied upon Mutasim and he was his successor.
The final alienation of Mamun was the people of Baghdad, by enforcing the inquisition/mihna of Mutazilism, which was extremely unpopular.
Eventually Mutasim moved the capital to Samarra to house his increasingly large slave army.
Samarra is a strange choice, lacking the major strategic and economic benefits of Baghdad and never became very large. imo Raqqa would've been a better choice, as it would've given more abbasid control over Syria and Egypt, whilst also being closer to the frontier and harkening back to Harun arRashid.
The slave soldiers were an effective military force, if you kept their interests in mind. But when Mutawakkil came to power, he was against the slave soldiers. But he didn't have any powerbases to properly challenge them.
The slave soldiers couldn't speak arabic, were resented as 'Aliens' by the populace, and didn't have any trades or skills other than fighting to fall back on if they are no longer employed as soldiers, nor any family to support them.
Thus they were totally dependent on the Abbasids for their livelihoods. If a Caliph went against this, it would be an existential threat for them. Thus, as Hugh Kennedy mentions, they acted out of desperation to save their lives and murdered alMutawakkil and his successors.
This was until alMuwaffaq managed to win the loyalty of the Turks via his martial prowess. And started a 40 year resurgence of the abbasids. Until 908 when the court chose to place a minor as caliph for the first time in history, so that they could manipulate him and become incredibly rich in the process. Resulting in the end of the abbasids as a proper force by the 940s.
Throughout all this time, they were being paid salaries directly from the state, not iqtas.
The Iqta system was a devolution, which came about during the Buyid era. This was since the abbasid treasury was near empty due to the chaos of the early 900s, unable to pay it's troops.
So instead, the iqta system have troops the revenue of a certain area of land. Unlike European fiefs, these were not hereditary and soldiers didn't rule these directly, only receiving the revenues from them.
This system continued for centuries until the modern era.
So, China. I'd argue that for the Seljuks to overtake China like they did Persia, there would need to be a parallel reliance on Turkic warriors, and later bureaucrats and scholars. This'd lead to a process of gradual Sinicisation which would parallel the Islamisation/Persianisation of the Turks IOTL. I don't know enough about mediaeval China, but as far as I know, Confucianism places a lot of emphasis on propriety and the correct (i.e., Chinese) way of doing things. The Mandate of Heaven -- the Confucian idea of legitimacy -- therefore requires state officials to revere the same traditions in literature, conduct the same standards of etiquette, and perform the same rituals to promote court culture. The Abbasid Caliph could promote a Turkish ghulam to a position of power -- a zealous convert who displayed his service to Allah, and is willing to subject himself to the Sharia, could be considered a legitimate agent of the Caliph; and over the generations, he and his children would become more "properly" Muslim, and more distant from the faith they converted from. But in a Confucian system, a barbarian cannot represent the Emperor, because the Emperor's Mandate is to rule in a civilised way. The mediaeval Islamic system allowed for Turkic outsiders to become minor power-holders and be promoted over the generations as they assimilated (and assimilated only in that they became Muslim; they remained quite distinct from Persians and Arabs). Meanwhile, the Confucian system expected bureaucrats and nobility alike to perform official culture as an entry requirement.
So, it'd be a lot harder for Turkic tribes to get a foot in any institutional power. This'd especially be the case if (as you propose) the Seljuks invade China having already been thoroughly Islamised. Though Islam has been a part of Chinese history since the Prophet (SAW)'s generation (see Sa'd ibn Abi Waqqas, one of the Prophet's disciples) -- it is still a foreign tradition to China, with its own ideas for political legitimacy and its own rituals for performing them.
Again, making some massive generalisations here. But you know, caffeine + hyperfixation + procrastination = lotsa semi-coherent writing.
I think a Qarakhanid conquest of China during 5 Dynasties and 10 kingdoms would be more realistic.
Since the Shatuo Turks had just formed the Later Tang Dynasty, with their first 3 Emperors being Turks. But they fell in 937. The Qarakhanid may be able to win over the Shatuo Turks and re-establish another Turkish Dynasty.
As i covered briefly in
https://www.alternatehistory.com/fo...stead-of-the-middle-east.547492/post-24720191
The difference between the Islamicized Turks and the Khitan/Jurchen/Mongols/Manchu is that these groups have no civilization to draw upon other than Chinese itself.
The Turks have the highly developed Islamo-Persian civilization and culture to draw upon. Of which the Chinese already recognised when Emperor Taizu appointed a Yemeni as head of the imperial Observatory in 961.
Thus Persianate culture would seep into a Seljuk/Qarakhanid China. Though the language would definitely remain Chinese.
This Persianate influence could be further increased if the Seljuks encourage/forcefully move Persians from central Asia to China, like the Mongols did. And give more importance and prestige to the Persians and Muslims than the native Chinese in the bureaucracy like the Mughals did in India.
Thus aspects of Persianate dress, cuisine, poetry, literature, music, philosophy, sciences and other aspects of their culture would likely affect the Seljuk imperial courts and general high culture.
From there gradually seeping into the masses.
But the sheer magnitude and greatness of Chinese civilization would generally make this not that significant....