But GrapheneOS relies on a proprietary, black-box security chip from Google... who pinky-promised to open-source it but never did, and that just doesn't sit well with me.

I think it's entirely possible that a compromised Titan module (whether such code ships with the device or is updated at a later point) could leak keys via some covert method, and possibly transmit via the baseband or through some other application/method where the OS is not really aware of what's going on.