Systemd - How do I automatically reload a unit, when another oneshot service is fired by timer?
Solution 1:
You can just add a deploy hook (not a post hook; you only need to do this if a cert is deployed) directly to the certbot configuration for your domain, in /etc/letsencrypt/renewal/example.com.conf.
In the [renewal] section, add a line like:
deploy_hook = systemctl reload nginx
That is all. You don't need to do strange things to the systemd units.