Work a while ago was started here: https://github.com/KDE/plasma-systemd-integration
It stalled somewhat. Partly due to some systemd things, partly due to the fact that some minor details which were missed due to the previous state of plasma startup.
I think our best bet is to work on this upstream rather than separately.
Aleix and I have been working on startup trying to make it possible, now startup is properly documented and the memory is fresh it's a good time to do this properly.
Gnome have since completed their port: https://blogs.gnome.org/benzea/2019/10/01/gnome-3-34-is-now-managed-using-systemd/
Behaviour has to be opt-in.
Current code is in davidedmundson/systemd_startup
though any ground work that could work standalone is being pushed directly into master.