The only immutable distro I've tried is Bazzite. It's gaming-oriented, but can work well as a desktop. It also does offer a KDE edition, but for some reason they intentionally ship with some GNOME programs instead of the KDE ones. For example, a fresh KDE edition installation includes Ptyxis instead of Konsole.
Also, DistroWatch's search has the option to only show immutable distros. It's under the Distribution Category option. I figure that might help you find a few more options.