> The security of the whole system is not compromised even though none of these modules is trusted, because all sensitive data is encrypted by the central MCU before sending it to a communication module.

No way. You need to decrypt that stuff before sending it to the communication modules (BT, WiFi, cellular, display) and you get them unencrypted from the same.

If the communication modules can decrypt the stream themselves, then eavesdropping will happen there inside.