That qt5ct thing , if you use a gtk based desktop is to setup qt theming, for qt applications is not needed for qt based desktops. Plasma-meta is a empty meta package with all of arch recommended packages as depency, plasma is a group to then you install a group of packagee diffete’t packagez belong 'to different groups
I usually install using the plasma group without a couple items (like Discover) and add a few extras like Dolphin, Konsole, Gwenview, Ark, and Ddialog. You can exclude packages by using ^ before the number listed for that package.
I just want to understand this last bit, so qt5ct is the package that controls qt applications/desktop environment, am I correct? And this is for XFCE, right? So I have to remove it because KDE uses somehting different?
Discivery is like the kde version of software. If you do pacman -Qi discover and add optional deps you can use it as software store but if you dont take tje optional you install things like sddm theme other kde stuf but i believe install such things already integrated in kde