While I like the idea behind GrapheneOS, I'd rather not place myself in jeopardy of some ridiculous charge like this one. I prefer to travel with a travel device, some inexpensive phone and/or laptop that contains nothing interesting. If they then wish to take it from me because I won't unlock it, then have at it! That said, the situation with respect to our Bill of Rights at the border has gotten ridiculous.
The "duress PIN that nigh guarantees destruction of evidence charges" functionality is extremely stupid, but otherwise GrapheneOS on a flagship phone is your best bet for an Android phone that can't be cracked by low-effort attempts, government or otherwise.