SUMMARY
The ability to configure and tweak plasma to our finest and most precises desires comes at a price: it's not difficult to break things. It is one of plasma's main critiques when it comes to ease of use for newbies.

While I don't have a magic solution to this general problem, here is what I suggest to improve one's ability to restore a broken KDE config to its defaults:

Change all of plasma desktop configuration's files and folder locations to a unique folder. This folder could then be subdivided with the different apps config files or folder. 

A discussion took place on that very same topic a few months ago on r/kde https://www.reddit.com/r/kde/comments/g1riaj/whats_preventing_kde_from_containing_all_settings/

Many good arguments were exposed against having a unique configuration file. How about one configuration folder to group them all? (no LOTR pun intended) as some suggest?

https://github.com/shalva97/kde-configuration-files groups all of the KDE-related configuration files, which is a great resource. If having one single folder is not feasible, could it be possible to integrate a "reset settings" button in the system settings app?

I am no developer, neither do I know much about KDE's desktop internal structure, please enlighten me if what I suggest is out of question.

STEPS TO REPRODUCE
1. Change plasma's configuration until the point where something turns out wrongly
2. Try to fix the problem by resetting the initial settings and empty the configuration files
3. Not be able to do it unless you know how to use the CLI and make extensive research on where is which configuration file

OBSERVED RESULT
KDE stays broken and users often install the system all over again.

EXPECTED RESULT
KDE's system settings offers a way to reinitialize every configuration file directly linked to the desktop. I am not talking about EVERY KDE app, only those who are related to plasma-desktop strictly. For example, KOrganizer or KMail are not compulsory/necessary to use the plasma desktop.

SOFTWARE/OS VERSIONS

Operating System: Manjaro Linux
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.77.0
Qt Version: 5.15.2