If you don't mind having some outdated packages (or using nix for the few you want to be most up to date) then you may find Debian works best for your needs. Manjaro's also a solid choice but the team behind it have been in some drama over the past few years so more people are saying to avoid it these days.

You could use Arch, its installation is nowhere near as difficult as people often state (it's also got much easier in recent years) but I don't think it'd fit your "it just works" needs. It does "just work" once properly set up, but the issue is making sure it's properly set up. If it's your first time doing that extra bit of configuration then you're bound to miss a few things

Manjaro! I can't run the KDE version myself, but I have both heard and seen good things about it.

Have you looked at tumbleweed? I've been using it without major issues for a few years across different devices. Perfect integration with plasma, rolling but stable distro, built in rollback feature, it's great

I've been a long time user of Debian + Gnome, but I've recently been using Fedora KDE spin as my daily driver just to mix things up a bit. I'd say Fedora is on par with Ubuntu with not having to tinker too much. The only thing I think I've had to really intervene with is getting the Nvidia driver going. Everything else I use just works, and there are plenty of packages available in the repo(s) for anything I'm not building myself.

As someone who can't quit KDE because of KDE connect, my go-to is debian. Debian 12 is an outstanding release, it's stable, and it works. The only gripe is that debian famously has later releases than most distros, which can be a problem if you need a recent version of say, go or rust (you can still install manually but apt exists for a reason), but in general it's not that bad and it's of course a tradeoff between recency and stability.

I've been using Fedora Kinoite and I love it. I wanted the same use case you wanted. Pretty much something that gets out of the way and keeps on working.

Too be fair, I was mostly inspired to use it because of the Steam Deck. Kinoite is an immutable OS, so it will prevent you from modifying core system files accidentally. It does have a small learning curve, such as learning flatpak permissions and using Distrobox/Toolbox to install cli applications, but I mostly use my computers as a gaming station where I chuck steam, a browser, and a few other tools in there and leave most things as default.

I always feel like my OS is "clean" despite having used it for a year, which is really cool. I'm curious what others might think of it as an easy distro though (it is for me, at least)

Kubuntu is fine if you don't mind the direction that Canonicla is heading.

Moving from Ubuntu to a debian based distro makes sense - a lot of stuff is familliar. Base debian is fine, but MX is a little more friendly. They have a KDE image here: https://mxlinux.org/download-links/