Tim, at this point my only recommendation is to run the “nuclear option” on the PGP keys, but please note that, this may bork your system.
From the Terminal, run the following:
sudo rm -rf /etc/pacman.d/gnupg
Reinitialize files & folders for keys
sudo pacman-key --init
Repopulate keys
sudo pacman-key --populate archlinux manjaro
Reinstall latest keyrings
sudo pacman -Sy gnupg archlinux-keyring manjaro-keyring
Refresh the signature keys
sudo pacman-key --refresh-keys
Now you can upgrade your packages
sudo pacman -Syu