I think what might have been happening is the phh GSI image I had silly-ly loaded to /system but didn't seem to work may have been partially booting, and giving the adb unauthorized error (with adb defaulted to enabled on that image).
Now that I've replaced system with the original image, I get the "starting up" boot animation but nothing further, it just spins endlessly. I've tried to boot into recovery every way I could find online, and none of them worked. I must have replaced recovery without backing it up, and don't have a way to get into fastboot or recovery to try to clear userdata or cache (if that's what is breaking the system and preventing it from booting).
I have
this arriving tomorrow (per your PM if I understood it correctly, maybe that will give me more options to see what is going on or get to the bootloader).