Please don't use Debian Testing. It doesn't receive security updates in a timely manner. I previously made a post about it:
Unlike Debian Unstable or Debian Stable, there is no fast track for vulnerability fixes needed into Debian testing. In my post above, I note that Debian stable and unstable received a fix for a very severe vulnerability. However, Debian testing was left to dry for almost a week, for a known critical vulnerability.
If you want more updated applications, you can use tools like flatpak, distrobox, appimage, or nix to layer newer applications onto a system of any Linux distro.
If you want newer base packages and underlying utilities, like a newer desktop environment, you can get stable distros that provide this, like Fedora, or Opensuse Leap.
Unfortunately it is still in Beta, but Opensuse Slowroll is OpenSuse's solution, a rolling release that delays package updates to test and iron out bugs before pushing updated versions out. It's somewhat of an in between a rolling release and a stable distro, and a very promising model.
You can also just directly use a rolling release distro. Debian Unstable isn't as bad as it sounds. Unstable doesn't mean unreliable or prone to breakages. Unstable just means that the bugs deal with will constantly be changing, instead of dealing with the same bugs all the time.
But please don't use Debian Testing. It's in the name, it's only for testing purposes.