OK, I got a cable and could connect the laptop to the internet in recovery mode. Then I followed the suggestions at https://salsa.debian.org/rpavlik/gcc-10-compat (mentioned here Upgrade to Byzantium) and I got past the libc6-dev message.
Patiently ploughing through the list of upgradeable packages using various iterations of apt install ..., apt update, apt upgrade, I arrived to the login screen and could login. System seems to be working, but new desktop apps need to get used to still.
However, the screen is flickering, last time I had this, I could cure it with an update of the firmware. Does anyone know if this is still the recommended way to go?