Well, the error you quoted makes two points obvious:
1. The current theme cannot be loaded
2. module "org.kde.kirigami" is not installed
You could try installing kirigami (from within a chroot environment) to see if that might resolve the issue.
sudo pacman -S kirigami
As is said, “nothing ventured, nothing gained”; however, the hard reset you described might have damaged anything; it’s difficult to predict.
Usually, in similar situations we might suggest working through a tutorial such as Recovering from an Interrupted Update/Upgrade, the general gist of which seems not unlike what you have already tried, yourself.
Mentioned earlier was to run the update again from chroot; this can’t hurt. As you seem to be more inclined to use pamac-cli, you could try the following:
pamac update --no-aur --force-refresh
and if that succeeds;
pamac update --aur
(if you have installed software from the AUR) and;
flatpak update
(again, optional)
Reboot and see what the status is then.