Does anyone have any tips on how you can still make progress in a language even though you are already fluent? I have this issue with English, because I'm at a point where I feel quite confident about my level yet I feel like I could still make progress, and I just don't know how. Is anyone going through the same thing, and what do you do in order to keep learning?
One of the things you could look into is reading high-school literature in English. You can search "high school reading list" to see what books are popular, and they provide great resources for practicing your inference skills and building even more vocabulary.
Here's some that I like (and have also read myself lately)
Zen and the Art of Motorcycle Maintenance
The Adventures of Huckleberry Finn
Heart of Darkness
I could go on forever, but anytime you read literature at the native level you will be reading the language in a whole new way :)