Many thanks @jeff - the problem was a path inside the systemd unit for the service following the Node move.
ExecStart=/usr/bin/node /usr/bin/ghost run
Path /usr/bin/node no longer existed (confirmed via ls /usr/bin/node). In case others have this problem, find node by using whereis node:
manager@server:~$ whereis node
node: /usr/local/bin/node
Then use a text editor to correct the config line:
ExecStart=/usr/local/bin/node /usr/bin/ghost run
Systemd then needs to be told about the changes, as the unit has changed on disk, by using sudo systemctl daemon-reload.
Ghost can then be started using ghost start or sudo systemctl start ghost_localhost.
Thanks for the help!