In order to use kde plasma on a Raspberry Pi, you either:
1) Use fedora 30 kde plasma spin
2)Install kde plasma on Raspbian lite.
Raspbian full doesn't support sessions, so use something else.
Also, when using kde plasma with Raspbian lite, be sure to switch off OpenGL in raspi-config (Advanced -> Experimental OpenGL or something like that).
As soon as the DM smiles, you know it's too late...