Currently I'm finishing The Return of the King (huge fan of the movies too). I do sometimes find the prose lackluster but not in a literary sense - more like it's not in alignment with my expectations of fiction in the context of current times. I want to dive deeper into characters' psyche and emotions etc. But you have to read it with knowledge of it's original release date.

Also, because you have seen the movies, you know how expansive and immersive this world is. Now try writing about this amazing world, encapsulating one specific story within it. Or try writing about our world and capture all of its intricacies. Wouldn't be easy, right? I'm just happy Tolkien managed to somehow describe it, maybe not in the most easily available or modern way (for obvious reasons), but he put this world in our heads, where it kept expanding further on. That's another reason why it seems to lack something - seeing the movies, we have let our imagination run wild as if we already lived in this world. So when the author doesn't describe this exact feeling/ connection that we have with it, something seems off.

Also, I would argue that Tolkien's old-school grand/ melodic writing is the actual source of the Middle Earth's atmosphere and it was written like this completely on purpose. What you have seen in the movies, is the vision that Tolkien showed to it's producers, through these exact words. Author took his world seriously and so did the reader.