I can report that I was able to fix my issue today!
I was trying to update to the latest release and I got puzzling errors with mismatching ghost-cli versions.
The solution was to remove the ghost and the ghost-cli folders from /usr/local/ as explained in this post: Updating Ghost CLI to get Ghost v3 - #12 by melphinstone.
After that I changed the binary in the systemd file to point to the right ghost (similar to this Problems starting Ghost 2.25.1 on Debian 9.9 - #9 by joncojonathan).
After that I reloaded the systemd daemon and ran ghost start, everything went OK.
I think this was some weird issue with mismatched ghost and ghost-cli binaries between users. Not really sure what the real culprit was but I’m glad it is fixed.