GrapheneOS supports up to 128 character passwords to support using diceware passphrases. Using a strong passphrase avoids depending on the secure element. A random 6 digit PIN is secure due to the secure element rate limiting. Only a total of 20 attempts are permitted so even a random 4 digit PIN would be fine.

GrapheneOS adds the option to set a 2nd factor PIN for fingerprint unlock to make using a strong passphrase convenient without the downsides of biometric-only unlock.

Pattern lock strongly encouraging using only a tiny subset of the possibilities so it's much worse than your analysis shows. It was removed from GrapheneOS years ago because it's far worse than simply generating and using a random 6 digit PIN despite appearing to be similar. It gives a false sense of security and we didn't want to add support for a duress pattern or random pattern generating alongside those planned features. Built-in random PIN and passphrase generation is still in progress but has been started and will be shipped.