While your blogpost isn’t completely right, it’s also not completely wrong.
You can absolutely customize image based distros, just as much as package manager based ones. You just need to do it from upstream, to modify the image itself, not from bottom-up like usually.
uBlue is the best example. There are already hundreds of available customized images around, including for Hyprland, Deepin, and much more.
That’s why immutable is often considered the wrong term for it. Image based, or atomic, is way better fitting.
One of the biggest pros, apart from the lack of maintenance needed (updating, etc.) is the reproducibility.
It’s very similar to Android, where every phone is the same.
Therefore, every bug is the same too, which is why the devs can roll out patches that fix everyone’s install at once.
Also, every update is basically a “reinstall lite”, so no package drift occours.
This makes them way less buggy in my experience.
I used the normal Fedora KDE spin for example, and after a few months there often came weird bugs that only affected my install.
Since the time I use Atomic, none of those problems came back.
Even if you decide to utilize BTRFS-snapper, which you suggested, the underlying system drifts apart from the original install.
Also, instead of Kubuntu, I would recommend the Fedora KDE spin or just Debian with KDE, if you really want to use something traditional.
I just don’t see any reason to not run Kinoite compared to a non-atomic distro, and it will only get better in the future.