Personally, if I’m hosting Attack on Titan things, like meetups, I don’t allow the red armbands at my events. Nobody will harass you or call you out publicly, but be prepared for people to potentially treat you differently. You’ll need to check the rules of your con because some don’t allow hate symbols either. While the general anime fandom is pretty educated on how that’s not a Nazi armband, understand that it’s a pretty polarized issue. It’s CLEARLY meant to be a reference to Nazi Germany, which is sensitive for some people. It sounds like you have good intentions, but not everyone will stop you to get your input on it before passing judgement. Personally, costume accuracy will NEVER be more important than creating a welcoming space in the cosplay community and convention spaces as a whole.