Not enough, e.g. when crossing the Russian border (even as a citizen) your phone can be connected via USB to a device that uses exploits and whatnot to download all of the data. Surely US border guard can do something similar.

And using encryption will only make you more suspicious, and may be a reason to get jail time until the situation is "cleared up" one way or another (e.g. by getting even more jail time).

Only a second phone works, if you can make it seem like a device you're actually using (though also not a silver bullet as e.g. a lot of messenger metadata is available to governments and border control can physically coerce you to log in to your real accounts)