Please consider switching to sudo apt full-upgrade. Full reasoning here:
https://source.mnt.re/reform/reform-handbook/-/merge_requests/22
There are regularly problems reported here because of it like Apt error upgrading tonight - #10 by murph
Glad to hear that you managed to fix the problem in the end! If your system had not booted, you could’ve upgraded the firmware from a vanilla system image on sd-card.