I wonder if despite our large vocabulary, English speakers tend to use a small subsection of it because there are so many non-native speakers. It seems to me from reading and watching people from decades ago that the language was more complex. (I don't mean this as any knock on people who speak English as a second language.)

We don't use most of them because no one could learn them all. There's more than 100,000 hanzi recorded in Chinese. Some quick googling suggests educated Chinese people know 8,000, and the average Chinese person knows around 3,500.

So 3–8%.

Nothing to do with pretention and everything to do with impossibility :D

Sounds... load bearing. Worse than being called pretentious, someone's gonna come along accuse you of being an LLM if you use words that aren't part of the normal lexicon.

Utilizing a breadth of vocabulary won't evince your being an LLM. None have such a habitude. You'll merely attract invidious barbed tonguelets.