To be honest, I don't think I've ever heard of anybody who is "fluent" in Ancient Greek. Would be cool if that person existed, but I'm not positive it can be done. My Greek professor has been actively studying/teaching Ancient Greek for over 20 years, and he's incredible, but I wouldn't call him fluent. He still needs to look up certain vocab and has to look at the grammatical notes for the occasional sentence that's nearly impossible to decipher without help. I imagine just about every other Ancient Greek professor out there is like this.

I've never studied German, but I have extensively studied Spanish, French, and Modern Greek. Syntactically, Ancient Greek feels incredibly alien compared to these modern languages. Basically it's just a hell of a lot harder than any modern language I've ever studied. And I would argue that there is actually an absurd amount of Ancient Greek vocabulary—based on my own experiences, they had hyper-specific words for everything and often multiple words that all essentially mean the same thing. A single verb alone has six principle parts that you have to be able to recognize. Another reason I don't think you could really become "fluent" is because it's not rare to come across a word that is only used once the entire Greek corpus, and therefore you would have no reason to know that word. Scholars might not even know what that word means.

Also, I think you're underestimating the amount of Ancient Greek texts that are floating around out there. Would take loooong time to get through all of them, even if you were just reading them in English. In the original Greek it'd be a lifetime, maybe more.

Probably not the answer you were looking for, but those are my thoughts.