Hello,
Moved to a dedicated topic so you get the proper assistance.
Make sure you do not have a mirror that is not responding. To change mirrors you can run from terminal:
sudo pacman-mirrors -f && sudo pacman -Syyu
You can try the interactive way:
sudo pacman-mirrors -i
and you chose only the mirrors you want.
Make use of Mirror-Check Service - is always mentioned in the announcements of the updates, depending on what branch you are.
Once all is updated, reboot and if SDDM still fails, try again my suggestion: