Hello and welcome!
I did like you a few years ago.
forged24 has already given you relevant information.
If you install XFCE4 and add Plasma, your system will be less responsive (I experienced it).
Imho, it's better to install one or the other (unless a little slowdown doesn't bother).
Personally, I use XFCE4, but I add dolphin, because for my use, I prefer it to thunar:
sudo pacman -S ark dolphin kdegraphics-thumbnailers kimageformats oxygen-icons qt5-imageformats
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Settings to make if you prefer a clear style (Adwaita) :
If after installing Artix XFCE with Adwaita theme, the background of Dolphin, Gimp or some configuration windows is black, just uninstall some packages:
sudo pacman -R artix-dark-theme artix-desktop-presets artix-gtk-presets artix-icons artix-qt-presets
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If on Dolphin, which I'm adding to XFCE4, the icon background is black instead of light gray, with black text :-/
As a workaround, edit ~/.config/kdeglobals and edit edit:
[Colors:View]
BackgroundNormal=220,220,220
mousepad ~/.config/kdeglobals