I finally succeeded to solve my issue and get back to a functional system.
Here is the commands I finally use from the live ISO to get out of that situation, based on the Arch Wiki https://wiki.archlinux.org/title/Pacman#Pacman_crashes_during_an_upgrade :
- First mounted system and efi, then /proc, /sys, /dev
sudo pacman --sysroot /mnt -Syudid not succeed so I had to follow https://wiki.archlinux.org/title/Pacman/Tips_and_tricks#Reinstalling_all_packages with the following command
sudo pacman --root /mnt -Qqn | sudo pacman --root /mnt pacman --overwite=* -S -
I also reinstalled kernel
I still had some broken packages, plus additional kernels, but I could reboot to the main system, update GPG keys with pacman-key --init and pacman-key --populate archlinux endeavouros, and clean the old and unused kernels with kernel-install remove
I may have to check if some damaged packages still remain though…
Et voilà ! A bit painful though for a first time. This can be closed !