I really don't understand dbus.

I think systemd targets work opposite to your expectation. The Wants in [unit] define the things that that unit needs to already be available. For instance, you might add Wants=network.target to the unit for nginx so that it won't try to start until the network is available. When I wrote a unit to start my company's application, I also had Wants=postgresql.service to ensure that the database came up before the application. Remember that sysyemd tries to run as many things in parallel as it can. This is one thing that makes it much faster than classic sysvinit which started things sequentially. But it means race conditions can occur. You use Wants to break those races where necessary. The targets that you'd specify in WantedBy in [install] more closely resemble SysV runlevels. You might want to read how runlevels used to work in SysV, in order to understand systemd targets.