@AndreasDimo since you are asking several questions it looks like you maybe asking for a recipe and not simply just suggestions. That’s ok. Here’s my recipe. Note that I went one step further and removed lightdm and replaced it with sddm.
See:
https://endeavouros.com/docs/pacman/how-to-install-desktop-environments-offline-install-users/
Install EOS Xfce via the install medium that you created.
Install plasma and sddm
sudo pacman -Syu plasma sddm sddm-kcm
Enable sddm to start at each boot
sudo systemctl enable sddm.service --force
Remove xfce4 and lightdm display manager
sudo pacman -Rsc xfce4 xfce4-goodies lightdm
Reboot
Use yay to install octopi and trizen
Set up octopi to also use AUR via trizen (Tools > Options > AUR)
Then using octopi hunt through:
kde-applications
kdeadmin
kdebase
kdeedu
kdegames
kdegraphics
kdemultimedia
kdenetwork
kdepim
kdesdk
kdeutils
for additional items that you may want.
It looks complicated, but its really easy. And leaves a fairly clean kde/plasma system. BTW if you are wondering about partitions, I have /, /home, and swap partitions for ease of changing and reinstalling distributions at the drop of a hat.
May I add that all of this information is available in forum posts and the wiki.