Ufff I'm rather fed up with code for a few days young padawan hahaha. Seriously though, the Qt5 documentation is outstanding and spot on. (hmm...my "state of documentation" thread comes to mind...lol) And in the end this is all QtQuick/QML which is the dev friendly frontend to Qt's C++. I also made a very customizable inline clock widget for KDE that did use Qt and it's way more complicated if you're not used to C (which I'm not).
From there code just tends to build up a lot. This started out with some very basic tags but I added all the blurs and positionings with several config values each, which called for a lot of {if else}s etc which bloated the code in the end. QML is like a combination of HTML and CSS. Really nothing to dislike about it actually! If I didn't customize all the QtQuick controls and had no blur at all it would be about half the code or less.
P.S.: If you're not a fan of blur and weird positioning, the non-blur centered configuration is really sweet. (sugar candy!)
Just set these accordingly:
FullBlur="false"
PartialBlur="false"
HaveFormBackground="false"
FormPosition="center"
and choose the background of your liking either through system settings or the config file. And if you love your image but it's got too much contrast variation you can dim it with DimBackgroundImage="0.5" (0.1 being very subtle and 0.9 being almost blackened out)
Actually that's the configuration of the video preview now that I think of it.
I would love to see a configuration GUI dialog possibility for the sddm-kcm module so users don't have to go into theme.conf manually but I suppose that won't get integrated any time soon.