And so much time is wasted in places like r/agnostic having the same "talking past one another because we're using different definitions of atheism" over and over and over.

I've seen the same thing happen in conversations about free will, racism, gender, and many other topics. I've learned through experience that there is absolutely no point in arguing about definitions, because, as you say, they're subjective and completely arbitrary. As such, there is never going to be a 'right' answer, but people are still going to argue about it like their definitions are objective truth.

So I say fuck all that nonsense. Instead, I try to cut to the chase and just straight up ask people, 'what do you actually believe', and let others who enjoy pissing into the wind argue over arbitrary terms?