Debian

In theory yes, but KDE is honestly the buggiest part of my Arch Linux system. It's not called krashes for nothing.

Honestly it's better to just go all the way and switch to something like Opensuse Tumbleweed (or Slowroll), Fedora, Arch Linux, or Bazzite (KDE version forgot the name) if you want a newer KDE.

The important thing to understand is that unstable doesn't mean unreliable. Unstable just means that it changes a lot. The bugs you get change, whereas Debian freezes things, meaning you have to deal with the same set of bugs, which can be important if you care about the system being predictable.

I have another comment if you are interested in mixing and matching packages from different releases/distros though: https://programming.dev/post/53832740/25029283 , although this might be a lot of work to put it together.

Yes, you can run Debian testing rather than Debian untestable.

Try Neon

What I found does not work great, is mix all the different channels and pull in something big like an entire desktop environment. It's possible. But it'll require manual work and occasional fixing once it breaks. Switching channels entirely seems better to me, I've ran machines on testing or unstable before. And while it's not recommended to do it, it does work just fine. Just be aware of the consequences. And mixing stuff works if it's just one or two specific packages, like the latest kernel.