I don't think that systemctl is what you want. That one is used to start a service or some type of app that will run in the background. The majority of the processes started by systemd (which is what systemctl manages) are run by root, and only in special situations, they are run by the user.
I'm not familiar with fuidsynth but it seems that it's a program with a graphic interface, which systemd is not really your best option. I would suggest you use startup application, so your program will start after you've logged into your user's account.
Here's a link with instructions on how to do it on KDE (which I'm not familiar with, I use Gnome):
https://www.addictivetips.com/ubuntu-linux-tips/autostart-programs-on-kde-plasma-5/