IANAL. This is my understanding.

For US citizens entering the US: If you're concerned about protecting your data from this threat, you should use GrapheneOS (or less good would be a latest iPhone in lockdown mode) and not talk to the police. The 5th amendment does not seem to have an exception at the border, so you can't legally be compelled to hand over your password. They can lie, they can detain you, they can seize your stuff. You shouldn't be the one arguing with them, you should be silent and ask for an attorney. As a US citizen, you can refuse and still enter

GrapheneOS has demonstrated to be robust against physical access even in AFU [1]. It's still a good idea to shut down / restart because BFU is much more secure. To maintain convenience while avoiding reliance on the rate limiting of the secure element, you can use a long random passphrase as your primary unlock (you enter it every time the device is in BFU) and a fingerprint with a second factor pin as the secondary unlock.

Even if you factory reset your phone and backed up data (to an E2EE cloud provider, over Tor and memorized the credentials), you should still not disclose your password because it could allow evidence to be planted or your device to be compromised. Nothing you say to the border agents can help you in court, it can only be used against you.

The duress pin is generally not how you should handle a situation like this. You do not need the duress pin to prevent access, its purpose is if you're being physically coerced as well as to give the attacker the dillema of whether to enter a coerced password (it even gives this benefit for people that don't use the duress password).

Sure, if police are behaving illegally and the information that would be disclosed is worth the possible punishment, then that's a personal decision.

[1] Based on leaked Cellebrite documentation, GrapheneOS was the best performing device against AFU attacks. It also has a customizable reboot timer to bring the device back to BFU after a set time of not unlocking. The options are between 10 minutes and 72 hours of unlocking and 18 hours is the default iirc. Apple and Google likely copied but without the option to customize. Theirs are set at 72 hours.