Kubuntu -> Manjaro KDE was a transition I made when I was still distro-hopping. I used Octopi as a pacman GUI which was super easy to use compared to Kubuntu's. Now that I've learned how to use pacman, I've still never had issues with a -Syu on either my Manjaro i3 or Arch systems, but I don't use Octopi anymore.

Depends on the WM you're using. I think (?) there are gui frontends to pacman? Never bothered to use them. But gnome is available on it, same as ubuntu. Of course, it's almost certainly a little less friendly, but probably not much.