I'm sure this is a noob mistake: 15.3.1, blinking login display, resetting PRAM unsuccessful

Noob mistake: I didn't turn off automatic updates. 15.3.1 installed, and seemed to go ok until it did the final reboot. Now at the login screen the display turns off as if the macOS windowing system is being reloaded. I don't mean like it's just a black screen, it's as if the entire video signal is lost. The screen will come back on for about 2 seconds before it repeats. During this small window I can move the mouse, I can type, but as soon as the screen goes off again the mouse location resets & the password field is blank (which is why I think the windowing is being reset; like restarting Finder).

I tried to reset the PRAM both connected to a USB 3.0 hub and directly through a USB 2.0 port, still no luck.

I can hold "Option" and get the boot picker, though, so it seems its definitely related to the latest OS update.

Any ideas on a fix that isn't an OS reinstall?

Thanks everyone.