iPad does not have enough storage to update. Photos taking up 14GB of local storage despite being set to optimize storage, has no way to flush its cache so the device can update.

Obvious solution: disable iCloud Photos. “Do you want to remove all these photos from your iPad?” Yes. Photos storage starts *increasing*, balloons up to 18GB.

Now fucking what?

@shac Many years ago my answer to that was to delete cache folders manually. But then Apple improved security and there's no jailbreaks anymore.