Thank you! Also thanks to nieldk! It works.

The systemd service was running right away. No further action required. Not sure if this was from finishing the post_install command error-free by the method quoted above instead of means by sudo. Or if for instance my previous attempts to get Prey to work with nodejs from the Warehouse repository were responsible.

This time one of nieldk's builds of nodejs from the Mer repository was used: https://build.merproject.org/package...latest_armv7hl

There seems to be an issue with Prey 1.3.9 reporting locations: https://github.com/prey/prey-node-client/issues/95

Installing Prey 1.3.6 worked however:

Additional feedback on Prey versions 1.3.7 and 1.3.8 would be most welcome. If someone could test a fresh install from Prey version 1.3.6 upwards and report on its behaviour related to the systemd service it would also be great.