Nix support is built-in to SteamOS already btw, I used that to set up Ship of Harkinian for example.

Could you elaborate? Does steamOS ship the nix binary & mountpoints?

I meant they have set up the Nix directory so you can write to it without having to mess around with bind mounts, overlayfs, etc. because the system is normally read-only.

So all you have to do is install Nix as a user.

IIRC, the nix package manager can run entirely user-level on any distro. It doesn't ship on the Deck, but it's the same process there as anywhere else.