As a language enthusiast who speaks ~5 languages at varying levels, I can relate to the feeling of "I am fully capable to have conversations about almost anything, but I don’t feel fluent at all". What I'd ask is: what exactly do you mean by fluent? In my opinion and experience, fluency is contextual. It's very hard to apply it in a generalized manner.
I'll give an example. I studied German in college, got to C1, and am comfortable reading/discussing the news, my hobbies, going out, day-to-day affairs. I learned a lot of my vocabulary by reading newspapers, and was regularly discussing geopolitics. I had no problem making friends in Germany, and I did not feel anxious speaking German. However, I would have trouble watching TV kids shows or spending time with a child in German. I have no idea what a lot of animals are called, or vegetables I don't usually buy. I've never learned how to say "You have sleep eyes" (or the local equivalent) or "Your shoelaces are untied" because it was never relevant to me.
On the other hand, growing up in an English-speaking country in a Cantonese household, I can say all those things in Cantonese. I'm very accustomed to a domestic setting in Cantonese. But politics? News? As if.
Folks often equate "Fluency" with "Speaking/understanding the language like a native", and I think this is a very unhelpful false equivalence. Comfort in a language across all situations and aspects are simply a function of luck and time, and folks in their native languages have simply had more time and more chances to be lucky. You can absolutely be fluent without equalling a native speaker, and honestly if you can have a conversation about almost anything, you're fluent in my books.
If you're looking for actionable items though, I'd advise thinking about why you feel non-fluent and being more focused. Is it humour? Watch comedies. Is it domestic conversations that won't come up by watching the news? Kids books, children's shows, parenting podcasts. Is it politics and general affairs? Newspapers. "Just speak with natives" is good advice in general, but it's not going to address your pain points unless you make intentional efforts.
You will get these things over (a long period of) time, but these are ways you can be more specific.
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EDIT: I'll add one last thing. In some ways, fluency is confidence. Even native speakers will reach the age of 50 and not know the meaning of common words. The difference is, they'll have the confident response of just shrugging it off and learning it (or not!) because they feel ownership of the language. Meanwhile, a non-native learner might think it's their fault and something they HAVE to learn. I think this is why a lot of non-native speakers actually have much larger vocabularies than a lot of my Canadian friends :P