KRDP: https://invent.kde.org/plasma/krdp

But it needs a logged-in session, you can't get access to the login manager.

I want to connect to a logged-in session; I can't get krdp to let me do it without me locally granting permissions though.

They fixed that. I can definitely connect remotely in an unattended way as long as I'm logged in locally.

These might have something to do with it:

https://develop.kde.org/docs/administration/portal-permissio...

https://invent.kde.org/plasma/krdp/-/merge_requests/102