I tried to utilize the following service files for running runst automatically via D-Bus activation but couldn't access the DISPLAY from systemd unit:

/usr/share/dbus-1/services/org.orhun.runst.service:

[D-BUS Service]
Name=org.freedesktop.Notifications
Exec=/usr/bin/runst
SystemdService=runst.service

/usr/lib/systemd/user/runst.service:

[Unit]
Description=Runst notification daemon
Documentation=man:runst(1)
PartOf=graphical-session.target

[Service]
Type=dbus
BusName=org.freedesktop.Notifications
ExecStart=/usr/bin/runst

Couldn't get this to work. It needs more investigation.