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.
What do you mean with “Operating System”?
It is most often installed as Docker container, which isn’t an OS, but just includes all dependencies to run. You still need an OS (like Debian) as host.