Nix's official package repository, Nixpkgs, is the most up-to-date repository out there[1]. Especially so for the unstable channel. So I assume it would fit your needs well.

But with Nix, you can be more flexible and mix packages from stable and unstable channels very easily. In my case, the kernel and desktop environment comes from the stable channel while the rest comes from unstable.

[1]: https://repology.org/repositories/graphs