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.
@shac same. I also had 10GB taken up by Messages media. But a button to remove all, or, say, older than 30 days is nowhere to be found. You can "review attachments" and then delete one by one. But there's hundreds…
@nyan_satan I definitely tried the reboot to clear caches and APFS snapshots. No difference.