timer are not supported, nor socket… only dot service files
please rather use cron, or you are on your own to have it working.
you need to create the entry in config db as you mentioned and you need to make it WantedBy= sme-server.target.
To do that you need to create a folder /etc/systemd/system/servicename.service.d
and add a file in it 51custom.conf with content:
[Install]
WantedBy= sme-server.target
if you were to do a contrib you would create the dot d folder in /usr/lib/systemd/system