I was crossing into Canada and 3 agents demanded my phone and password (as well as my SO) under threat of detention. They took the phones in the back for 45 minutes before they came out. Asked questions about a conversation I had about marijuana with a friend months ago, basically accusing me of being a drug dealer when even in the most wild reading of that conversation, that cannot possibly be the case. Anyway, who knows what happened in that 45 minutes, probably our data is still sitting deep in some Canadian government server somewhere.

Cheap Android phone. Temporary, cheap SIM (or none—you can get one where you're going) with a different number from your real one. Only way to go. Bonus that it's 100% reasonable to do this because you don't want to risk having your main phone stolen/lost, too, so it's not even a weird (suspicious) thing to do.

Eventually, this might be seen as non-suspicious. Maybe. I hope so. It's quite prudent.

BUT for now it's also something only a very, very small percentage of the population would consider doing and then actually do. Border agents would know this, empirically. So if you're doing something that's highly unusual for the general population then you are, unfortunately, doing something weird and suspicious.

TL;DR: it doesn't matter that it's reasonable; it's still fringe and, sadly therefore, suspicious

The government has everything you put on Facebook already. And everything you backed up to Apple or Google photos. And your emails.

If there's something else they're interested in, don't carry it over the border.