I am not sure what the state of your network will be after removing and reinstalling networkmanager. You might have to start/restart the service in there.
Make sure you have a copy of the networkmanager package in your cache. If not, download it before you start.
Also, this is a pretty minimal kde install. You will probably want to add applications to suit your preferences after that.
For me, personally I found it was cleaner to just do a fresh reinstall when trying to go from one of the big DEs to another. There’s just too many little bits.
But if you want to try @dalto’s got the right idea
Yes, basically all the gnome config in your home directory will be left behind. The package manager won’t touch any of that.
There is no doubt it is cleaner to reinstall. Alternatively, after removing gnome and installing plasma you could either create a new user account or purge everything from the existing account.
If you want to pull in many of the KDE apps, you could also install kde-applications (which includes Dolpin and Plasma).
If you’ve set any overrides for QT theming, eg using qt5ct, in the GNOME environment, you should remove those because KDE will conflict with them, and you’ll get all your QT apps looking janky
When i installed it the Arch way my pacstrap included only base, base-devel, linux, linux firmware, plasma, sddm, firefox, nano, man, amd-ucode, grub, efibootmgr, konsole, dolphin