(No natural speaker, sorry for that)
Same here with a Quest 2 (I know other device but the symptoms are precisely the same). The device switched to boot slot b after showing some red warning about untrusted firmware. (you can check the boot slot in the device info (the only menu item still working).
It seems like the update was not counted as successful, so the device decided to boot from fallback b which doesn't seem to work/exist? Problem is that you can't switch back to the primary boot image without an unlocked boot loader. I fear this situation would be only solvable unlocking the device for which you need the unlock code which is unlikely to get from meta.
If for any circuumstances your bootloader is unlocked you can set the device back to a which might make the device functioning again or at least might allow to come to a state where adb/sideload is working to reflash the firmware.
Notably fastboot reports that it should be possible to switch the slot but fails on try with some sort of access denied.
I already tried to write the support attaching full technical reports and logs but got only a text snippet about rebooting the device. This is frustrating as there are at least good chances to unbrick the device by just switching the boot mode. If anybody knows another way I'd really appreciate that.
