[foresto]

This makes me wonder how you installed KDE. If it was by installing a single top-level package (e.g. kde-standard or maybe kde-plasma-desktop), then I would expect apt autoremove on that package to just work. (Unless you later manually installed more packages that shared some dependencies with the top-level package, in which case you would have to remove those as well.) APT distinguishes between manually-installed and automatically-installed packages for exactly this reason.

It also distinguishes config from other files. By default, it will keep modified config files when you remove packages. That way you don't accidentally delete the custom web site definitions that took you hours to create. You can use --purge to override this conservative behavior.

Debian's packaging system isn't the simplest, but I find it to be reliable, good at dealing with edge cases and failures, and extraordinarily well documented. It's part of why I switched to debian-based distros well over a decade ago.