So, i just brutish removed lightdm sudo eopkg rm lightdm, created the ~/.xinitrc file with cp /etc/X11/xinit/xinitrc ~/.xinitrc, added dbus-update-activation-environment --systemd DBUS_SESSION_BUS_ADDRESS DISPLAY XAUTHORITY and exec i3 at the end.
Logged in from tty, issued startx and... it works! No delay for starting caja and pavucontrol anymore.
So it seems like dbus-update-activation-environment --systemd DBUS_SESSION_BUS_ADDRESS DISPLAY XAUTHORITY really does the trick. But I would really like to incorporate this into logging in with lightdm. Any ideas where to add this command?
EDIT: Not sure if it actually worked or my mind was going crazy, but it does NOT work anymore after I rebooted.