@manton Issues like this are why I'm grateful Backblaze exists.
I recently tested syncing a single image for TIL with iCloud and it reacted as if I'd thrown at it a collection of those high-resolution images taken by NASA. This was just for between the M1 iMac and iPhone, on a stable connection with no other items being actively saved. That's both the beginning and end of me trusting Apple for important syncing.