I recognise this problem (and I also really want a copy of your game; a sensitive thoughtful take on the history of the American West sound like just my cup of tea).

I love writing history-themed games, but the fear of getting it wrong or being guilty of appropriation means I mainly stick to European history, though that’s not my only interest. I’m scared to reach further afield. Assuming my situation is fairly typical, this is not a good outcome for widening the variety of RPG settings out there.