I used Kubuntu for years, but ultimately moved away from the Ubuntu based distros due to Canonical cruft. I haven't really missed anything going with vanilla Debian.

I'd recommend Fedora KDE, it's vanilla and well used enough to be able to easily find answers.

https://www.fedoraproject.org/kde/

Ubuntu based distros are fine too, but there are a few weird things to get to grips with like Snaps.

There's really not much difference between most distros these days so I'm sure if you like one you'd like the other.

I've found Gentoo Linux to be a good developer- and sysadmin-oriented distro. It requires a lot more work up-front than most any other distro but -IME- once you have it running, it just keeps running and upgrading just fine. If you wish, you can even subject yourself to systemd, as that's a supported init system.

As a bonus, if you don't want to build everything from source, there are prebuilt packages available. Instructions for how to use them are in the "Installing the base system" section of the Gentoo Handbook. I've not used the Gentoo-provided prebuilt packages, but I do use my own prebuilts. I've found the process of using them to be well-documented and fairly straightforward.