So, I ended up leaving the laptop on all night so it could get this bloody thing done. Now, on all previous updates, it's waited until I'm in the middle of something (unsaved, of course), and rebooted out of the blue, without warning. This time, it sat there all night and did nothing for about 7 hours, waiting for me to click the button to say Reboot.
It then took about 40 minutes to restart but it now appears to have updated.
I like the new dark mode, but once again, it reset all my fonts, which is annoying. Had to come to work so I've yet to explore any other changes.