It's highly unlikely that the Canada Border Services Agency, of all organizations, is able to defeat competent modern crypto implementations.

It's also quite unlikely that they can circumvent a basic `tar -c /dev/block/* | gpg -se ...` scheme that's trivially doable on unlockable Android phones. (Replacing the bootloader with a maliciously crafted variant that returns false partition data after bootup would work, but once again: CBSA.)

You're probably safe if you just do an iCloud backup and wipe followed later by a restore.

Certainly do replace the SIM card as soon as possible though, IMO. Unless you can verify it somehow.

If you are asked to provide your password, encryption will not help you.

Perhaps still better than trusting border patrol with physical access to the device.

No, if you trust the encryption (which implies the firmware), you don't need to trust the ISP or storage provider. That's the whole point of encrypting stuff.

Interestingly, in the case of handing over your phone, there are some odd quirks in security that make things a little topsy turvy. Normally, while the iphone has better device security, in the case where you are conceivably worried about firmware hacks (to the degree you think it's possible), the iphone presents as a juicier target, as the hardware is far more homologous.