I'm new to the whole speedrunning thing, but is it possible to create categories by difficulty? What about the cameo ending in the arena?

It is possible to create categories by difficulty. I have already seperated NG+'s difficulties. Unrelated to this topic. Could you try to use splits or a timer for your next submission? Downloading and using Livesplit or wsplit would help us mods out when you make a claim to a time for your submitted run.