The easiest one-click-and-go solution for a laptop with dual-graphics is probably popOS since system76 is a linux laptop manufacturer themselves, they put a lot of work into it “just working”.
It isn’t KDE by default, but you can just install any DE you want, the OS doesn’t really care.
As popOS is Ubuntu based it’s not rolling, but you can enable the devel repos which effectively makes it a rolling release. There’s also scripts like Rolling Rhino that do that for you and take care of some caveats.
In case that you have the necessary skills or are eager to learn you could install Arch yourself. It would be rolling release and you would be able to choose the desktop environment you prefer. The Arch wiki also explains the support for the XPS 9700. I was used to using Linux before, but needed about 2 weeks and several installation attempts to get Arch working correctly. I am now very happy with it. Manjaro also has a version with KDE Plasma if you are not sure if you are able to setup Arch.
I’m with Sapiens on this and suggest Arch. I’m also suggesting it because it’s not too hard to get up and running through the installer they made. It messes up some things, like not importing wifi settings you input during installation or following the locale you set. But those are very simple things you can change once you booted up the system.
Turn key solutions are, as always, PopOS! or Ubuntu (which should even be available through Dell themselves or have some kind of special support since I’ve seen XPS machines come in an Ubuntu variant).
bluetooth doesn’t work before login so I can’t use my mouse to login. This is especially problematic since I need to use the on-screen keyboard to login. I can’t physically use the touchpad or keyboard even though they work.
Display scaling resets on reboot
The external 4k display via USB Type C to HDMI is flaky causing the display to lose connection and sometimes cap out at 1080p.
I have a similar laptop but its a Precision 7550 from Dell and I run Manjaro on it. Sound and blutooth fired up out of the box. Getting the nvidia/intel GPU switching has been a bit harder.
as far as I know, the only linux distro that was ever tested with the new xps line was ubuntu and even then there’s no official customer support for it
given the killer nic, that alone, kinda makes it a terrible option for a linux build