When i learned German what really helped me improve past the beginner/intermediate phase was to start reading books. I still didn't really understand the grammar theory (still don't know what a genitive is) but after a while i found i could just "instinctively" use the correct form...
At least if i remembered the gender of a noun, and that was the part that took the longest because a) there is no rhyme or reason (for the most part) and you just have to learn them, and b) my native language is also gendered so i would frequently make incorrect assumptions based on what felt "right" to me.
I'm not taking any courses right now in the languages i'm learning but i may try it in the future if i can find an in-person course.