Thank you for this, I didn't know this service existed, however I do NOT agree with your statement old fashion 'ntpd'. The man page of systemd-timesyncd itself says: This minimalistic service will set the system clock, which might be sufficient for most users, the more robust implementation of the NTP deamon might still prove to be of value, and makes life consistent for people managing more than a single Linux system, systems that don't always have the systemd-timesyncd service.
The systemd-timesyncd service is an SNTP client, only implementing the client part of NTP. If you want to use your pihole as the source to go for time (NTP server), this will NOT be possible.
The systemd-timesyncd service, according to the manual, writes to tour SD card, every time the clock has been synchronized, NOT one of my favorites to read, I rather spend the 8$ for a new SD card (mentioned in some other topic) on something else, but that's a personal point of view.
Thank you again for pointing out the existence of this, running out of the box, but unconfigured service
If somebody decides to go for the NTP daemon instead of the systemd-timesyncd service, here is how to clean up, avoiding the service to run but do nothing (as you explained earlier)
sudo service systemd-timesyncd stop
sudo systemctl disable systemd-timesyncd