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This question contains two very large assumptions which, when checked, unfortunately reduce it to nothing as far as I can tell.
The first is about the clan structure in Confucianism. As one of the comments points out, in Confucianism the clan is more important than anything, as can be seen in the Analects:
The Governor of She said to Confucius, 'In our village we have an example of a straight person. When the father stole a sheep, the son gave evidence against him.' Confucius answered, 'In our village those who are straight are quite different. Fathers cover up for their sons, and sons cover up for their fathers. in such behaviour is straightness to be found as a matter of course.'
This is precisely the opposite of Roman law, where one of the earliest and most influential legal precedents is the case of a judge prosecuting his father according to the letter of the law. Indeed, as you mention intra-clan marriage was common in ancient China, and even today clan associations retain some limited power in China (much reduced after a century of authoritarian nation-state operations).
Your second assumption is about "secularism", which you associate with individual religious liberty. This certainly did not exist in ancient China. Something like this secularism emerged in Japan in the Edo period, around the 17th-19th centuries, but while there was Confucian language involved it didn't derive directly from Confucian teaching. What you're looking for in terms of tolerance of different faiths had nothing to do with inter-clan marriages, but was a cautious and only halfway agreement between feudal lords, which some of them rejected in the 17th century with pro-Confucian, anti-Buddhist campaigns. It seems your association of secularism with individualism is not quite right.
Furthermore, the modernization of Japan, which spurred its economic growth, began completely contrary to the thesis you're thinking of, with a wild anti-liberal movement destroying Buddhist temples. The arrival of secularism in China was of course with the Communist party wreaking destruction on the country's traditions in a similar way.
I recommend the following publications for analyzing the problem of secularism.
I don't have any recommendations about secularism or clan structure in China -- my apologies