I'm pretty sure Debian's default desktop environment out of the box is Gnome, but like any decent distro you can usually either get it preinstalled with different environments, choose it during installation (I think it's part of tasksel on Debian) or changing the desktop environment should not be much harder than installing the appropriate package or metapackage, typically on Debian this is something like task-kde-desktop but there are more details for KDE specifically here. PikaOS likewise has a KDE installer image.

I think guides are even better on Debian personally but YMMV. I understand the attraction of having a good ecosystem for stuff like that, and Debian is itself very popular, maybe not quite Ubuntu level, but being the upstream for almost every apt/deb based distro means that there is usually pretty good compatibility between guides for different flavors and often the Debian one is where things get started and you can often use Debian guides directly on Ubuntu or PikaOS with no issues and no modifications needed. Occasionally you might need to tweak some package names or paths slightly, apt-cache search <name> or dpkg -l <package> can often help a lot with any inconsistencies you might encounter.

The reverse is often also possible: I know I have probably used Ubuntu guides directly on Debian in the past, although I can't remember the last time I actually did that, because the Debian-specific guide is usually what the Ubuntu ones are actually based on and I can't remember finding a Ubuntu guide where I couldn't also find something corresponding for Debian. Sometimes I do kind of look at them both and try to understand if they're doing certain things differently for some particular reason and that helps me understand if there might be version or library issues that might be something to keep an eye on.