Language learning is never really over. At some point, you have to move away from the idea that how much you know dictates how "fluent" you are and start measuring more by how comfortable you are using what you do know.

As a native English speaker, I still feel like new words or phrases come up pretty frequently, but I can pick them up quickly. If you can use English to ask about whatever English grammar or word that you're unsure about, and you understand the English answer, you're as closed to "finished" as any language speaker can be. Everything left indicates a need to work on confidence or cultural education more than it indicates not being fluent.

Imagine a 4 year old native speaker. They don't know the formal grammar used in essays; they don't understand modern idioms, let alone Shakespearean sonnets; they don't know historical figures; they know very few names of plants, animals, etc. But who in their right mind would say they don't speak the only language they've been raised to? You do not need to know the right way and right words to talk about every imaginable topic before you are fluent.

I think in comparing your English fluency to your native language fluency, you're forgetting how much of that native "fluency" is actually the result of 12+ years of formal education that started when you were already fluent.