So unfortunately the premise of your question is somewhat off, as most of the factors you ascribe as being absent weren't, but I'll try to address some of those.

To start, the split was anything but "down the middle". There were Confederate sympathizers in the North, and staunch Unionists in the South. This of course is not to even mention the border states where strong factions existed for both sides, and in particular with Missouri, resulted in an especially bitter guerrilla campaign of neighbor against neighbor. This older answer focuses on the aspects of disunity within the southern states, and while not the focus, this answer on massacres of POWs does touch on just how awful Missouri was in the war.

Insofar as religious dimensions existed, the it is not something I'm well read on, but there absolutely were denominational splits which can be seen. This answer is focused pre-war, but in particular the follow-up does illustrate how religious denominations themselves fractured over the issue of slavery, and while it is one of those books that sits on a far too large reading list, God's Almost Chosen Peoples: A Religious History of the American Civil War by George Rabe is one I've heard good things about on the topic. As for ethnic dimension, sadly, I have no past things to link for that, but it too you can find plenty of fodder for too with immigrants and draft riots and so on.

As for foreign intervention, while it is true that no power entered the war as a belligerent, the overseas dimension was quite massive, as touched on here, and as noted here there was quite a lot going on that was directly related to the war, with France in particular trying to capitalize on American weakness.

And to be sure, the country was reunited, but to wave that off as having happened in a simple manner is certainly off, as it took decades, and generations, for that to happen and plenty would argue that the wounds still remain not fully healed. This answer looks at folks who refused to make peace in the immediate aftermath, while this looks at how commemoration of the conflict changed over time. I have a few that look at 'Lost Cause memory' and this one I think is particularly interesting and this one too.

Now, to be sure, none of the above is to say I don't understand where you're coming from! There absolutely is a narrative of the Civil War which does feel 'simplistic', but that is a factor of how we're taught many things. You learn about the Civil War in school and it needs to be a streamlined narrative for pragmatic reasons, and if you don't go into deep self-study, the complexity likely won't be that apparent. But you can expect it will be similar for how the English Civil War is taught in British schools, or the Spanish Civil War in Spain. All of them are going to have their simplified narratives that elide over certain complexities in the interest of being able to teach it as part of a class that likely needs to cover several hundred years of history! That doesn't mean any of them aren't complex, or lack nuance, and the American Civil War is no different.

/u/Georgy_K_Zhukov already dropped in several good answers of theirs, but I'd just like to add this one of mine that focuses on Missouri, a border state, before the war -- it gets into the complexities of a state that had a majority-Southern-born population but one that was seeing growing immigration from the German states, many of whom had abolitionist or at least anti-slavery leanings.