My Debian Buster has a systemd service named
rc-local. It is launched after the networking target and runs
/etc/rc.local start if /etc/rc.local exists and is executable.
You could put your code in /etc/rc.local, make that file executable and ensure that
rc-local.service is enabled.
Code:
echo rclone mount --allow-non-empty Polaris: /mnt/Polaris >> /etc/rc.local
chmod +x /etc/rc.local
systemctl enable rc-local
In case /etc/rc.local exists already, first check what's inside.