I think the issue with making successful Civil War movies is that people prefer going to movies to have fun. The Civil War is mostly going to be pretty bleak (not in a fun way) if you represent it accurately. People would rather see Dwayne the Rock Johnson drive a muscle car through a building than see something poignant, morally ambiguous, and probably fairly grim. If you do make a fun Civil War movie, it can very easily come across as distasteful for any number of reasons.
I can imagine doing something like O Brother, Where Art Thou, where you have charming screw ups bumble their way through the war. In order to do a serious movie, you'd have to make heroes everyone likes, and villains everyone hates. No matter how you cut it, that will be hard. Plus the overseas markets which are playing ever larger roles may have little interest.
Maybe the story of Junius and Albert could be converted? Just kinda dumb down/spiff up the main characters up a bit. Cast likable leads...?
https://www.amazon.com/dp/1610393791/?tag=civilwartalkc-20