< Long Term Servicing Conundrum
As you mentioned liking KDE: If you have reasonable hardware ressources (and for Windows you've probably run more than necessary), I'd also recommend using KDE as a Desktop. Like any other desktop (Mate, XFCE would be my next choices) it`s available for a ton of distros, so the freedom of choice is just another kind of freedom here, distrowatch.com may help you with that. And most distros could be started and run in virtual machines for a first evalutation.
My personal experience with Kubuntu was really bad (but should be re-evaluated again). So although I like debian based distros I've spend most of my 20 years of linux desktop usage on OpenSuse. To make it a grandpa tells from the great war story: I actually bought my first Suse-Linux in a box filled with a bunchfull of floppies and a book ...