# /etc/systemd/system/zfs-load-key-all.service

[Unit]
Description=Load encryption keys
DefaultDependencies=no
Before=zfs-mount.service
After=zfs-import.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/bash -c '/usr/bin/zfs load-key -a'

[Install]
WantedBy=zfs-mount.service