The reason most distros don’t do this is because they usually try to steer users to their preferred desktop. I don’t mean that in a bad way, but often they call it the flagship edition. This is usually the desktop environment that they develop and test. Think Linux Mint with Cinnamon, popOS with Cosmic, Zorin, etc. Pretty much every other distro that has options do exactly what you are showing through the use of the venerable calamares installer that you see in your screenshot.

Dolphin should be qt6 for quite some time now. So you should need qt6ct. Also I’m not sure if either qt5ct or qt6ct is maintained. Their is a fork in the aur that I have been using https://aur.archlinux.org/packages/qt6ct-kde, but not sure what to do outside of arch.