How can I recover forgotten Macintosh HD key on MacBook Pro Max?

This sounds like the Recovery Key for FileVault would be required to get things right again

Protect data on your Mac with FileVault

Excerpt from above

• iCloud account and password: This choice is convenient if you use iCloud or plan to set it up—you don’t need to keep track of a separate recovery key.

• Recovery key: The key is a string of letters and numbers that’s created for you—keep a copy of the key somewhere other than your encrypted startup disk. If you write the key down, be sure to exactly copy the letters and numbers shown. Then keep the key somewhere safe that you’ll remember—but not in the same physical location as your Mac, where it can be discovered

WARNING: Don’t forget your recovery key. If you turn on FileVault and then forget your login password and can’t reset it, and you also forget your recovery key, you won’t be able to log in, and your files and settings will be lost forever.

Use Disk Utility to erase a Mac with Apple silicon

If you cannot recall your Apple ID then it must be reset with a new, different ID. Click on the link below and follow the process. Then write the information down and save it in a safe place for th next time it is needed.

Change or reset the password of a macOS user account - Apple Support