I usualy do full system backups with
restic backup -q --exclude="/var/cache/*" --exclude="/proc/*" --exclude="/sys/*" --exclude="/tmp/*" --exclude="/var/tmp/*" /
depending on which linux flavor you use, you maybe have to exclude some other virtual directories. Or you take a look at the option --one-file-system, which excludes directories like this automatically. But think twice about that, because it also ignores other mounted filesystems that you might want to include in your backup.
Regarding a restore of such a “whole system backup”, I wrote something about that some time ago here: Bare metal restore from restic repo: worked fine!