Establishing a full DE on Arch is as simple as 'pacman -S plasma kde-applications sddm' followed by 'systemctl enable sddm.service'.
Compiling everything manually would take a long time. You would need to work out all the dependencies, make everything, install everything, create all the configuration files... You'd then need to do this again every time a package is updated. You'd hope that every dependency had an RSS feed you could follow. For those that didn't you'd need to regularly check for updates. Arch requires two commands...
I've done all three. Arch isn't on the same level as Ubuntu, Fedora, or Manjaro on level of ease for instillation.