Whenever i've tried KDE my favorite experience has been on OpenSUSE Tumbleweed. Its a rolling distro, but provides a really nice installer and stable experience because of their automated QA process. The KDE stack is provided as intended by upstream without much customizations (they use their wallpaper for the lock screen and desktop), and the updates are quick. If you try it, you might also like their support for btrfs + snapshots. If an update isn't to your liking, it'll allow you to boot into an old snapshot and rollback.

I still use tumbleweed, but with Gnome and this has been my distro of choice for the past couple of years. (I moved from Ubuntu)