Yes clearly once you have a key logger installed you have been compromised and have to assume the worst. But it would still be really useful to know that your keystrokes are being observed and recorded. I wonder if you could make a little dongle that would encrypt your keystrokes as they leave your keyboard and have them decrypted via some “Secure Enclave” on your device’s CPU, which can only be accessed by the lowest level (signed) kernel code.