I am currently experimenting with this. I dont know what the best solution is. I will add a new post about this in KDE Discuss and Lemmy.
system or user services?
common directory?
order of launch
what are the dependencies and what depends on them
It is pretty crazy that entire KDE Plasma doesnt use systemd, and I can now (after looking through /etc/xdg/autostart add geoclue, baloo and orca to the possibly unwanted processes).