I also made a very similar comment, but with uBlue (Bazzite, Aurora, Bluefin) instead.
They are still pretty vanilla, but include a big list of QoL stuff added in, like staged updates, Distrobox, a huge list gaming tweaks in Bazzite, and much more.
It’s basically stock Atomic made right!
I’ve used them for a year now, and they’re fantastic!
Just a small heads up for OP: You have to do quite a lot of (advanced) things differently from now on if you choose Atomic. Use containers (Distrobox, etc.) for everything you can, avoid installing stuff on the host if possible, etc.
I wanted to add my own comment first, but yours covers it very well.
OP will be very happy with KDE and its’ app ecosystem.
About distro choice, it doesn’t matter very much.
My top recommendation would be Fedora Atomic KDE (Bazzite or Aurora), because it “just works”, has zero maintenence, will never break, and especially on Bazzite, gives you a first class gaming experience.
Other than Bazzite/ Aurora, I can of course recommend classic Fedora KDE, because of the sane defaults and both modern and stable release schedule.
OpenSuse Tumbleweed would be great if OP wants something that’s both bleeding edge and reliable, and EndeavourOS if they like Arch with very sane defaults, but more minimal than Tumbleweed.