Hong Xiuquan’s story is pretty bonkers, but I think the more sensationalist aspects of it overshadow the possibility that some sort of disruption to the tottering Qing was due in that era. Increasing foreign entrenchment with defeat in the Opium Wars, an increasingly sclerotic government, and much local turmoil (even prior to the rebellion, there had been ethnic and class-based revolts in Guangzhou, iirc, and some of Hong’s support base was because he was of the Hakka minority) all made the situation fraught.

Hong being influenced by American Christian missionaries is odd, but there is much precedence for rebel leaders who took on dynasties to be of millenarian, even foreign beliefs. Throughout Chinese history, the White Lotus society, with millenarian Buddhist beliefs, had challenged the Qing and the Yuan (the Ming founder was a member). The Yellow Turbans of the Romance of the Three Kingdoms fame were Daoist esoterics. Manichaean rebels fought the Song. I think a lot of times it’s just that religious cultists had the necessary social structures in place to launch rebellions, and the cohesion to continue the fight as opposed to scattered peasant revolts. The history of Chinese dynastic shifts is replete with secret societies. The Taiping were noteworthy not only because they were Christian-influenced, but because it was an act of such an organization being created in real-time, out in the open.