can’t you delete some files to make some space?
the pacman cache, for instance?
it lives in:
/var/cache/pacman/pkg
sudo paccache -rk1
or
sudo paccache -rk0
to free it
or
sudo rm -f /var/cache/pacman/pkg/*
to just delete it
or
sudo tune2fs -m 1 /dev/sdxx
to be able to use some space previously only accessible to root
or
sudo journalctl -b | grep sddm > myprotocol
(which should get around the reserved block percentage because it is run as root)
quite a few options …