CBP is the one who would be interested in your phone. TSA doesn't care unless it has an explosive taped to the bottom.

I think Apple gets this a little bit more right. A hold of power and volume up locks it and requires the pin to unlock.

Android has ROMs that have stronger features, but basic Android is lacking in this area.

E.g. CalyxOS allows you to put the phone in "Before First Unlock" mode. In which even if extracted only has system data, not user data. GrapheneOS uses even stronger protections, it lock downs the whole kernel.

This isn't the intended purpose for this app, but it can sorta work for your needs.

https://play.google.com/store/apps/details?id=vikesh.dass.lockmeout

You can set up the thing to lock on a schedule for one minute. If you don't mind being locked out for a minute occasionally while you're using your phone, it effectively means that if enough time passes your phone will have locked and you'll need to use your passphrase.

Edit: anyway the existence of this function means it's possible to write an app that does what you want.

Hold power button for 5-10 seconds to reboot.

I dont have a lock down button to press on mine. How did you get that?