A bit late to the game, I know, but now that /nix is included by default on the Stable channel of SteamOS 3.5.5+ have you run the single-user installer for that?
Do you know where the configuration.nix file is?
Preconfigured /nix is just a bind mount from /home/.steamos/offload/nix to /nix, but that should make it single user nix installation and use work by just following the usual instructions without need to touch steamos-readonly-settings.
Personally, I just moved store end var from my previous nix mount directory, everything remained working and now SteamOS updates no longer remove /nix, because it is there by default.