What you could do is set up a chroot jail and use debootstrap to create a bootstrap system. Then put the sources.list for Debian Sid and run apt install kde-plasma-desktop
Mount /proc and /dev in the chroot jail.
Then go to a text terminal (alt-f3) and stop your login manager service.
Chroot into your debian sid chroot and start the x server with start (I think)
These steps might not work but it’s kind of a roadmap of how i would do it.
What you could do is set up a chroot jail and use debootstrap to create a bootstrap system. Then put the sources.list for Debian Sid and run apt install kde-plasma-desktop
Mount /proc and /dev in the chroot jail.
Then go to a text terminal (alt-f3) and stop your login manager service.
Chroot into your debian sid chroot and start the x server with start (I think)
These steps might not work but it’s kind of a roadmap of how i would do it.