Xfce does not get updated often, so by that definition it is the most stable.

It's also pretty stable from the perspective of not crashing, but I can't say I've had much trouble with KDE either.

This has to be a troll post

You can pick any bleeding age distro which allows locking your package versions, but as others have said, you might be asking for competing ideas.

However, if you have time to spare and want a smooth forehead: NixOS. You can install the stable version and then declare that every other package you install comes from the unstable branch.

sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixpkgs-unstable

/etc/nixos/configuration.nix

{ config, pkgs, lib, ...}:
let
   nixpkgs-unstable = import  {};
in
{
  # Stable X with KDE
  services.xserver.enable = true;
  services.xserver.displayManager.sddm.enable = true;
  services.xserver.desktopManager.plasma5.enable = true;
  # other packges from the unstable channel
  environment.package = with nixpkgs-unstable; [
    firefox
    chromium
    libreoffice
    # ...
  ];
  # The rest of the stable stuff
}

https://nixos.org/manual/nixos/stable/#sec-installation

I am already sorry for the pain you will have to endure trying to find the proper documentation, but I truly think this is the best solution for your usecase. Good luck.

Feel free to ask for help on Matrix chat or the forum.

Again, apologies.

XFCE works for me, but I've heard that LXDE is pretty good too.

Xfce. I have used it daily for at least 14 years and it has never crashed on me. I often forget it is even there, it is so reliable.