Don’t know about Cachy. But the general way is to just install KDE and then change where you want to go in your login-manager. You might have to disable automatic logins to get that.
Many distributions have some kind of meta package that installs everything KDE related called kde, kde-desktop or plasma-desktop.
After everything is running how you like it you may remove all Gnome packages and install sddm instead of gdm as a login manager. But that’s optional.