The Qing were isolationists
You're essentially falling for propaganda, first created by the British to further their imperialist ends and inherited by Chinese reformers and nationalists. The Qing were among the least isolationist of major Chinese dynasties. Although the Manchus made serious efforts to destroy foreign trade in the first years of their empire, during the bloody struggle with the Zheng state in Xiamen and Taiwan, once the Zheng state was no more the Qing were quick to restore foreign trade to its pre-Ming vigor. In 1684 (only one year after the conquest of Taiwan!) the Kangxi emperor allowed the Chinese throughout the four southeastern provinces to trade with foreign countries as they wished. This revolutionized Chinese trade. For instance, the number of Chinese ships visiting Nagasaki rose from 41 between 1680 and 1684 to
445 between 1685 and 1689. The following year, all foreign merchants - including those not from tributary nations unlike in Ming times - were able to visit some four customs houses (Shanghai, Ningbo, Xiamen, and Guangzhou) and 161 customs stations. The establishment of the Canton system changed this to some extent, but still, a tributary relationship was totally unnecessary to trade in Guangzhou. This is far superior to the Ming tribute trade system, as well as the Song system where only nine ports were prepared for trade.
To quote a 1751 Parisian encyclopedia,
At the present time... by opening commerce with other countries, the Chinese have increased the means of enriching their own. They now not only suffer, but encourage both near and distant nations... to come and trade with them; ad bring them the most valuable commodities; and, at the same time, allow their own people unto a great number of foreign parts... No wonder then that it is so opulent and powerful, when all the four parts of the globe contribute to make it so.
before there were the treasure fleets
Zheng He's expeditions were extremely anomalous and born out of the poor economic policies of the Ming, which suppressed private trade and attempted to expand tributary trade across the entire Indian Ocean. They were only active between 1405 and 1433, and the transcontinental reach of Song merchants or the economic transformations of Southeast Asia incurred by Qing merchants did not require any treasure fleets.
a neomongol dynasty for this
Manchus, not Mongols. Calling the Qing "neo-Mongol" is about as accurate as calling the English "neo-Normans." The Manchus and Mongols shared some of their lifestyle (although the Manchus were much more agricultural) and some cultural concepts (the universal khanate or the script), but they spoke different languages, had vastly different histories, and each conceived each other as alien. The Dzungars, for instance, called the Manchus "southern barbarians." The Manchu emperors' sources of legitimacy was also very different from those of Mongol khans; while they did claim support from God, they did not belong to an illustrious Chinggisid lineage and did not derive their legitimacy from their recognition by the Yellow Teaching in Tibet.
Also, it's ironical you claim a "neo-Mongol" dynasty would have nothing to do with "sea boats" when the Mongols invaded Java and Japan by sea and was generally extremely interested - if not successful - in maritime expansionism.
No way in hell will the Qing do anything that has to do with sea boats
Oh really?
"Maritime trade benefits the well-being of the common people" (
haiyang maoyi shi youyi yu shengmin) - the Kangxi emperor, 1686