Tune compton@.service parameters a bit.
Add a "primary-xserver" service/target.

This is for units that need "any" X server in their environment.

The primary-xserver@.service unit writes to a file and sets itself
Before=primary-xserver.target.
Those who need a DISPLAY should set themselves Require=primary-
xserver.target/After=primary-xserver.target and use EnvironmentFile=.
Use "primary-xserver" system in the dbus unit.

So DISPLAY-requiring things can now be dbus-activated (like add-
printer).