If you're on American soil they can just detain you. Or worse.

If you ever want nightmares, read the story of Maher Arar.

isn't the right move here: wipe your phone, travel to destination, then restore from cloud backup? in the middle, you can let them inspect your wiped phone.

Good luck getting anyone close to this to go on the record about it though given such things normally come with corporate or government gag orders.

There are hundreds of privileged vendor binary blobs on most flagship devices not even Google gets source code to though so supply chain attacks should be assumed.

Sure, the NSA can probably pull this off. Thing is, the NSA probably does not need to do this at immigration.

I seriously doubt that this is a realistic problem if your threat model is anything less than "The NSA is very interested in me". In that case I don't see how you could trust any phone, regardless of it having been in the hands of border officials or not.

Correct, and I do not own one.

Wipe it, let them inspect it, sell it, and buy a new one.