Startup
Startup can be summarised as being:
plasma-core.target plasma-workspace@.target graphical-session.target
plasma-workspace@ is the target explicitly activated.
X11 and wayland
plasma-workspace@ is a template file that ends with x11 or wayland. That will then require the correct kwin_ startup order can be different between the two.
Wants & Order
Note that in systemd dependencies (wants/wantedby) counter-intuitively do not determine order.
plasma-workspace wants graphical-session, meaning it will make it something started by it, but it also explicitly comes before graphical-session.
The order of events is: plasma-core does anything that adjusts environment variables plasma-workspace@ starts all runtime services graphical-session is at a point where everything including runtime services are up
Adding a new service
If it should only be used on plasma it should be wanted by plasma-core or plasma-workspace@.
That service is responsible for setting "After=plasma-core.target" if we need envs set up.