Create a bootloader recovery SD card using the Raspberry Pi imager https://www.raspberrypi.com/software/ - use a spare SD card and choose the desired bootloader (SD/USB/net boot) in the operating systems -> misc utilities section.
Hard to tell what went wrong when you updated, this is the first report of a failed bootloader update that I have seen.
so long,
Hias
Hias,
Thank you for the suggestion.
The bootloader recovery method did work for me (at the second attempt, tough).