You can declaratively manage your phone with something like github.com/devindudeman/nix-android so you'd point at a decoy config file and rebuild before crossing the border and then point at the up-to-no-good config file once across.

Of course you do have to be able to reference those files somehow, so they have to be online, but if there are no references to them on the device then the border folks would have to somehow know about them and be able to access them in order to link you and your device to the declared configs. There are a lot of ways to hide such a thing.

Presumably the most sensitive thing here is your social graph and messages. I haven't verified if nix-android handles them in a way that would make sense here, but it's at least plausible that it might.