In fact, there were different groups of Kazakhs opposing and supporting the Oirats. The last independent Oirat ruler, Amursanaa was supported by Kazakh Khan Ablai and even married to his daughter. Initially, Abai refused several demands of Qings to surrender Amursanaa for which they made several expeditions against Kazakhst. They also fought with Kazakhs after the subjugation of Jungaria.
So, that horseman, very possible, could be a Kazakh
No Qing Chinese painter ever surpassed Castiglioni (Lang Shining) in realism. He is truly a Renaissance Man transposed to China, and giving the touch of Da Vinci to traditional Chinese watercolor and miniature paintings.
However, it simply didn't catch on with the Qing painters. You need more theorists from Renaissance Italy to bring the whole system there. But it just died with Castiglioni and the closing of the empire to the West.
There are several more Castiglioni paintings of Mongol or Manchu "baturs" that are truly magnificent.
You also notice from the paintings, the Qing baturs' reliance on their Mongol-Manchu bow and arrows and the Kazakhs' and Jungaars' utilization of muskets of Russian or Ming origins.
The Qing baturs wore chain-mail armors just like their Central Asian adversaries, out of practicality. Later Qing ceremonial armors would discard such diversity, in favor of more decorated quilt armors (you also see more paintings of ethnic Han soldiers equipped with muskets). The Qing Eight Banners, from the paintings of Castiglioni, was truly a grey-clad, hardy army of hunters and nomads not unlike the Cossacks.
Use of firearms: the Qianlong period Battle of Isik-Gul (background: The Qing general Guwalgya Hurkyi pursued the Afaqid prince/Khojijan from Qarasu in Xinjiang to Badakhshan in Afghanistan. The battle Isik-Gul was decisive in Hurkyi's defeating the Khojijan. You can see muskets being used massively by both the Qing and the Afaqid Sarts. The Sarts wore hats that look somewhat like the Qashgari kalpaks. There might be a large Kyrgyz contingency in the Afaqid army. It could be why Khojijan was retreating into Kyrgyz areas in Afghanistan. However, Khojijan was most likely betrayed by a Badakhshan's Ismaili Tajik ruler Sultan Shah, who beheaded him and presented his head to the Manchus)