If we could put the DE onto a different layer we wouldn’t have to have different spins of fedora and when you’d want to use another DE you could just rebase to another DE instead of another full image
On the “immutable” Fedora editions Silverblue (Gnome) , Kinoite (KDE), Sericea (sway), and Onyx (Budgie) you can rebase the system to either of the other editions.
Did a quick search and found an example here https://www.makeuseof.com/rebase-fedora-silverblue-to-any-version/. See also How to rebase to Fedora Linux 37 on Silverblue - Fedora Magazine
Yeah as @augenauf already said; you can trivially rebase to any other image you want to.
sudo rpm-ostree rebase fedora:fedora/39/x86_64/kinoite, wait for it to be done, reboot and you’re in KDE.
There’s other work that goes into spins & atomic flavours outside of just installing a DE, specific apps, themes, tweaks etc are included too; there’s more than just dnf install @kde going on.