>>96398686
The anime anthology film Robot Carnival has a segment where two 19th century mecha battle each other. The fight takes an entire afternoon and both have a pretty hard time doing anything effectively, which is exactly how I like my fantastical 19th century technology.
I'd recommend it both as a good film and as a source of unit images or rules inspiration.
https://www.youtube.com/watch?v=id1eRjO4p_0

Starts around the hour three minute mark.