Just add a religious spin to the OTL Civil War. The ACW was more destructive per year when adjusted for population than the Taiping Civil War.
Ie the Tailing Rebellion saw 20-30 million killed yes, but that was over 14 years and out of a population of 420 million. That means China lost, on average, a bit under half a percent of its population per year. The US Civil War saw some 2% of the population die in the army, plus an undetermined number of civilians. That translates to somewhere over one half of a percent of the population per year.
The reason the Taiping Rebellion was so bloody was because the population of China was so huge and the war lasted over a decade, not because it was exceptionally worse than other such wars.
In terms of long term implications, though, the Taiping Rebellion was considerably worse.
For one, US came out of the war more centralized than ever while the Qing decentralized. There's the claim that the regional armies the Qing allowed regional governors to muster up were the precursors to the fully autonomous Warlord Cliques half a century later, as well. At the very minimum, the Qing authorized the creation of regional armies not under the control of the central government or military. And while decentralization isn't inherently bad, the Qing was trying to modernize and not get picked apart by foreign powers and internal squabbles for the next few decades, and that tends to work better with centralized nations (Japan's the obvious role model here).
Also, sure, only half a percent of population per year but the conflict also centered on the richest regions of China and the total war nature of the conflict meant a huge amount of arable land and wealth was burnt over and over the course of 14 years as land changed hands. Plus, the number of refugees also numbered in the tens of millions, which further disrupted internal stability and economics in the Qing Empire. Contrast that to the US Civil War, where the majority of economic power was concentrated in the North, which was not burnt to a crisp in total war. Also helps that the US Civil War, outside of Sherman's March to the Sea, was not really as focused on destruction so much as achieving victory, whereas the Qing and Taiping engaged in widespread massacres and reprisal killings. All that meant rebuilding would take far longer as well.
And, of course, the fact that the Taiping Rebellion was one of a number of rebellions happening at the same time affecting different regions, namely the Nian, Panthay, and Dungan Rebellions, all of which engaged in the aforementioned massacres and reprisal killings, and didn't really end cleanly. Taiping remnants would continue fighting into the 1870s in the border regions and disrupted the politics and economy of SE Asia, including Qing tributaries, as well. The US Civil War ended cleanly by comparison.
All of this is at least partially due to circumstances the Qing faced (domination by a hated ethnic minority that was far outnumbered by the ethnic majority, widespread famine and disease outbreaks and insufficient government responses and aid to them, a shortage of land due to a population that trebled in around 100-150 years) that the US certainly would not face. It was those circumstances that made siding with a religious madman palatable to those who just wanted to change the status quo, after all.