> would you recommend that over a distribution focused on rolling releases, like Arch?

Everything about Debian except the `stable` repository is explicitly a rolling release.

> Debian strongly advises against mixing repositories

Certainly you wouldn't want to add in the other repositories if you're aiming for Debian Stable type guarantees.

This is the `sources.list` file that I've been using for nearly a decade:

   deb http://deb.debian.org/debian/ testing main non-free contrib
   deb http://deb.debian.org/debian/ unstable main non-free contrib
   deb http://deb.debian.org/debian/ experimental main non-free contrib

And then I have a preferences file that prefers testing to unstable to experimental (actually three separate files in the preferences.d directory, but I'd think you could combine them.

   Package: *
   Pin: release a=testing
   Pin-Priority: 700
   
   Package: *
   Pin: release a=unstable
   Pin-Priority: 650

   Package: *
   Pin: release a=experimental
   Pin-Priority: 600

It may not be advised, but it works pretty well. Sometimes you have to get a bit creative when you go to run `apt-get dist-upgrade` and it wants to delete half your system, but usually you can just manually install individual upgrades (`apt-get install <x>`) until it unwedges itself.