Default devuan ascii comes with a “display manager” called slim Slim allows you to define an “autologin” user but it does not easly lend to start a plasma KDE session

A display managed that support autologin and select plasma.desktop is sddm You can install BOTH and then just declare that you are using sddm

you can define the autologin user in /etc/sddm.conf and if the file is not present you can create it.

I still have some issues with the reboot and shutdown command in sddm, but autologin to plasma works