So, I found a voodoo ritual to coax the device into recovery.
https://blog.tho.ms/hacks/2021/03/27...-into-recovery (2nd option)
From recovery, I got into fastbootd. But any reboot into system would just bootloop as before. After finally succeeding again with the ritual, I noticed that I was in slot_a... where I swear it was using slot_b before. So I activated slot_b and rebooted into the OS fully.
I tried to dump both boot_a and boot_b but their hashes were the same... which made no sense if it couldn't boot slot_a properly. Flashing magisk into slot_a and booting would get to the bootloader and then pause there with the fastboot channel immediately open. Flashing into slot_b and booting was the same result... stuck at the bootloader with only fastbogus able to make anything happen.
Of course this meant that I could no longer boot using any partition. Conventional wisdom said that if the original boot.img were the same for both slots, then something else was wrong. Nothing that I flashed to any slot would boot to the OS. So I had to figure out the URL for update.upx, decrypt it and sideload it using the recovery.
So, the device is technically working... but I'm not any closer than I was at the beginning of the day. Two things bother me... #1, fastboot boot .img didn't actually boot the image. #2, flashing the dumped image onto itself killed it instead of doing nothing noticeable.