In Bootloader mode only fastboot devices is working, that’s normal.
I’m busy at the moment and on a trip, so that I have no time to bring support or helping hands until second week in June. Sorry. And cannot be online all the time.
I would install the avb_key again.
Flash the image via sideload again.
Try to reboot
Or would try to change the active slot and install the image via sideload in the second slot, to be sure that it is synchron in both slots.
Until you do not lock the bootloader again, nothing worse can be happen.
Lock the bootloader only if get_unlock_ability is 1
Command (in Bootloader mode) to change active slot:
fastboot --set-active=a or
fastboot --set-active=b