Thanks to your hints I was able to get it working again. I went into recovery mode and tried to run “dpkg - Repair broken packages”. I kept getting the error “temporary failure resolving us,archive,ubuntu,com” (using commas because it doesn’t let me post a link, even though I’m not linking to anything). I eventually discovered that I had to run “grub - Update grub bootloader” first, and then dpkg was able to download and repair the broken packages.
It then booted into KDE using the nouveau driver. I then was able to open Software & Updates, go to Additional Drivers, and select the 570 driver. It was able to update it this time, and after restarting it’s working again.
I had a similar issue to this a few years ago when I tried to update my NVIDIA driver and that time had to completely reinstall Kubuntu. Is it a safer process maybe to first switch to the nouveau driver in Software & Updates, restart, and then switch to the new NVIDIA driver? It seems that KDE/Kubuntu has an issue going directly from one NVIDIA driver to another. Or is there another recommended way to do it?
Thanks again for your help!