Depending on where you live, you may not be required to disclose a passcode or alphanumeric password to law enforcement, but biometrics (such as your fingerprint) do not fall under these laws, and so you may be legally obligated to unlock it: https://en.wikipedia.org/wiki/Key_disclosure_laws
GrapheneOS allows you to set up auto-reboot, which will set your phone into BFU (before-first-unlock) mode, which requires your passocde/password, even if you have a fingerprint unlock set. You could set your phone to reboot every 8 hours. Another perk of this is that most malware cannot persist after a reboot due to verified boot—which is why advanced malware like Pegasus prevents you from turning your phone off and pretending it is off when 'turned off' by the user so it can persist. Though, if you have malware installed on your phone, you have bigger problems...
Additionally, biometric unlocking might be easier than you assume with a motivated threat actor. You can, for example, extract a thumbprint from a high-resolution picture. So, most politicians should probably not be protecting their phones with their own thumbprints. A thumbprint is technically more secure because it's more complicated than a 6-digit passcode. A 16+ character alphanumeric password would be the most secure option, but it is not convenient.
Another thing to consider is that the Titan chip inside of Pixel phones is designed to combat rate-limit bypass attacks. Cellebrite and other tools work primarily by trying to bypass rate-limiting. For example, if you get the pin code wrong 5 times in a row, you might not be able to try another pin for a time. Cellebrite makes brute-forcing attacks possible in a reasonable amount of time by disabling this limit. With a Pixel phone, the amount of time between each attempt increases after a certain amount of failures, until after 150 attempts, you can only make 1 attempt a day. More reading here: https://github.com/Peter-Easton/GrapheneOS-Knowledge/blob/master/GrapheneOS-Security-Q%26A.md#does-graphene-allow-me-to-nuke-my-data-if-say-the-phone-gets-ten-incorrect-guesses-at-the-pincode
So this makes using a short pin more secure on a Pixel phone, because the Titan chip is a much harder to defense to get past than purely software-based rate-limits.
It's not a clear-cut situation. You need to decide what compromises you want to make.