Anecdotally, I think a lot of my vocabulary came from reading older novels. J.R.R. Tolkien, C.S. Lewis, Issac Asimov, Arthur C. Clarke, Agatha Christie, Sir Arthur Conan Doyle, etc.

Recently I've started reading modern scifi/mystery novels and found the vocabulary to be very simple (smooth? Like the text is lacking texture) by comparison.

If you can't get through novels without falling asleep, try audio books (preferably unabridged). Lots of classics on https://librivox.org