I want to switch to another DE on Fedora, from KDE to COSMIC, is it possible to switch in some simple way, kind of with rpm-ostree or something? I’ve searched the web but only found info on doing this with Atomic desktops, but I’m not using one and not planning to yet


If you read the document there’s this GUI tool for the same :
https://docs.fedoraproject.org/en-US/quick-docs/switching-desktop-environments/#_using_switchdesk
Switchdesk is a GUI for changing the default desktop once they’re installed, not for installing desktops.
The top of the page you link explains how to install alternative desktops which is using DNF. The GUI route to do this would be to use a DNF frontend like DNFDragora and search for the relevant desktop metapackage, and then install that.
Got it.