It is great that you are approaching Koine Greek as a language, and that you expect it to have all the communicative abilities and learning requirements that you have come to expect in your study of Japanese.
Kaminokamen wrote:I say one thing, and someone says one thing and everyone's speaking Erasmian lol.
Yes, unfortunately that is something that is likely to happen, given that there are so many variations in pronunciation for Greek. Follow one system for your speaking. You will have to adapt to that in your listening skill, much like it takes time to listen to regional varieties of either English (and gender / regional varieties of Japanese if you've gotten that far in your studies).
Kaminokamen wrote:How do we, as learners of the language, incorporate all these words that didn't exist back in the day?
Quite often people use words from later periods of the lanaguage. You might like to use Modern Greek words that have been adapted to Koine phonology and grammar.
Acquiring vocabulary, even for the things that there are words for takes time (years and years of time). Sometimes, we can know theh grammar that we want to use, but not the word. In those cases, I sometimes imagine the English (or another language's) word as a short-term measure until I find what word is suitable.
Historically speaking, to arrive at Modern Greek, the speakers of Greek did two things. The first was to adopt foreign words. In Byzantine Greek, many more words were taken from Latin than we already have in the NT. Biblical texts borrow / adopt / adapt words and names from Hebrew / Aramaic. From the 13th to the 15th century and then later up till the beginning of 19th century many, many words were taken from Turkish. The other thing was to reuse Greek words from earlier periods in new or revived ways possibly with the same old meaning or maybe with new meanings - borrowings from earlier periods of the language have happened at some times during the evolution of the language.
Ὃς μὲν κρίνει ἡμέραν παρ’ ἡμέραν, ὃς δὲ κρίνει πᾶσαν ἡμέραν. Ἕκαστος ἐν τῷ ἰδίῳ νοῒ πληροφορείσθω. (Romans 14:5)