Do you have any evidence to support this?

macOS @ M1 boot process looks similar to that of iOS devices: https://support.apple.com/en-gb/guide/security/secac71d5623/...

Under the assumption that SIP is not disabled, and computer doesn't have any 3rd party kernel extensions installed (both of which are safe to assume for most people), it's close to impossible to inject something in the boot chain.

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I think the best possible option might be to replace the keyboard with the one with a hardware keylogger, and record the actual password.

Yes, that option with the keyboard would be the easiest option. See? The point on encrypting your whole disk is not to avoid tampering, but to lock your machine in public places and be sure that no one can access your data upon placing that disk in another machine, or even trying to boot it.

As always, security always operates with some threat model in mind. Of course some government-level agency with physical access and any amount of money to spend could likely get access to your device (though the usual $5 wrench is probably still the "better" way) at the price of a (or a few) zero days. But you are likely not a key person in toppling a government and it would be a giant waste on their parts.

The threat model the general populace faces ranges from something as mundane as your ex wanting to extract something from your device to basically being the know-all key to every important document, photo and data you might have. I think I can safely say that under this circumstances not even physical access poses a real threat to the device's security -- it is tamper-proof to replacing most critical parts and the boot-sequence is cryptographically secured. Biometric data is not stored in software, so not even that can be spoofed.