How do I learn a more "uncommon" language once I'm semi-intermediate?

Hi all! Sorry for bothering if this is a stupid question.

I'm a native English speaker who was born and raised in Czechia. I am still living here to this day. I didn't take Czech seriously as a kid, but nevertheless, I went to a Czech kindergarten and went through schooling in Czech. Currently, I'm taking it very seriously and genuinely want to improve.

However, I've found myself in the "intermediate slump." I can't seem to improve at all, and all that has happened is that I'm more aware of the mistakes I make but can't express my thoughts any more eloquently than I could years ago. It's honestly hellish because even though everyone around me is very nice and supportive, I still feel stupid compared to them because of my struggles with the language. I got the grammar down, since I internalized it early on, but I struggle with vocabulary, slang, and idioms.

I tried reading. Children's books are too boring and easy for me. Advanced books are so hard it's almost pointless because I have to look up literally every single word I see. I also tried to read intermediate books filled with slang to hopefully internalize some of it, but I faced the same issue as I did with advanced books.

I tried Anki cards, but I didn't find any Czech decks that are more advanced. All I found seemed beginner-intermediate to me, and I already knew all the words in them.

Finding Czech content in general is hard. I managed to find one YouTuber I like, but I don't think that's enough to truly improve in a language. Czech people often favor English media because of the lack of good Czech media, and thus there's a lack of Czech media because no one's interested. 🥲

I have very academic Czech, but I have very weird gaps. I have gaps an low-intermediate learner wouldn't have, but I also have knowledge a high-intermediate wouldn't have. I feel stuck and don't know what to do.

Would anyone have any advice for me? Happy learning & good luck on your studies, everyone. :-)