Last time I tried krdp, it didn't fit my needs. I needed to have already started krdp locally if I want to connect remotely. Neither x11vnc nor freerdp-shadow have that limitation.
It is possible to start it on-demand via the command-line. That is how krdp is developed and tested. You will need to pre-authorize krdp to the portal system, though.
That's documented here: https://develop.kde.org/docs/administration/portal-permissio...
Hah, that actually works, thanks!
Is it possible to do the permission-set remotely?
If you can shell in, I don't see why not? It's a command-line thing. It's intended to also be usable for things like Ansible. If it doesn't work, please file bug reports!
Since Plasma 6.1 (june 2024) you get a system settings module where you can enable KRDP to run at login.
https://quantumproductions.info/articles/2024-06/krdp-plasma...
(I assumed being logged in is what you meant by having "a running session".)
That "with a running session" bit greatly helps here. While GDM allows you to have remote login and headless sessions [1], with SDDM you're locked out for the moment. There's plans to turn SDDM into a KDE-powered, more featured plasma-login-manager with KRDP integration, but no concrete development yet [2].
[1]: https://gitlab.gnome.org/GNOME/gnome-remote-desktop
[2]: https://invent.kde.org/plasma/plasma-login-manager