For that you would need to be able to fully trust your cloud storage provider, the ISPs used in transint to and from, the used encryption methods and last but not least: the baseband firmware of your phone.
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.