I think the Mongols should have a more reliable way to produce the Huihui Pao in standard multiplayer games, instead of only receiving it randomly from the Khaganate Palace.
This is also a gameplay issue. The Khaganate Palace already has heavy RNG, and getting the wrong unit at the wrong time can make the landmark unreliable. A Mongol player who specifically needs long-range anti-building siege should have a way to invest into Huihui Pao production rather than waiting for a random spawn.
To keep it balanced, I do not think Huihui Pao should simply become a expensive standard siege unit. A safer implementation could be one of these:
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If Mongols choose Khaganate Palace, Huihui Pao becomes trainable from the Siege Workshop in Imperial Age.
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Alternatively, Mongols can research an expensive Imperial Age technology, such as “Huihui Engineers” or “Persian Engineers,” which unlocks Huihui Pao production.
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The unit should have a high cost, long training time, and possibly a high stone or gold requirement, so it remains a siege investment and can not be spammed
This would reduce frustrating RNG and create a clearer strategic identity for Mongol late-game siege without necessarily making the civilization overpowered.