I can’t update my system because I get some variant of File /var/cache/pacman/pkg/systemd-resolvconf-257.7-1-x86_64.pkg.tar.zst is corrupted (invalid or corrupted package (PGP signature)). I’ve already tried sudo pacman -S archlinux-keyring which did not change my results. I also found this post and tried running everything reccomended there but still got the same results.
Then try to remove that package, download it again and install it.
Welcome to the forum 
Did you also try to remove the file ?
If you only want to remove this file without clearing the entire cache this is what you can do.
Open a terminal and type the following
cd /var/cache/pacman/pkg
Then try
sudo rm systemd-resolvconf-257.7-1-x86_64.pkg.tar.zst
Also you can empty the entire cache with
sudo pacman -Scc