I think you're missing my point. Worrying about metagaming is thing causing the pain. Encouraging players to just engage with the fiction relieves the pain. It's counterintuitive but works: the less you, as the DM, worry about metagaming, the less it actually shows up at your table. IME, at least.
Bad faith play will ruin any style. Try to separate the concept of metagaming (using player info to inform game decisions) from cheating (true bad faith stuff like buying the adventure path to read as a player, fudging dice, intentionally not recording HP or spells slots used) and you are part way there.
I am chuckling at imagining their faces when you mixed things up on them, though.