Update: As I wrote in https://forum.pine64.org/showthread.php?...2#pid39812 I was able to successfully upgrade directly from 16.04 do 18.04 without problems, just with some held packages, but later I was able to unhold all of them and KDE works. Did not try to update kernel and everything, will try it today.
edit: updated kernel and everything works
edit: updated kernel and everything works