Thanks for bringing the disko command to my attention.
However, since we are talking about installing NixOS declaratively, and it's done through nixos-anywhere, which will install it[0] to the ram unfortunately.
[0]: https://github.com/nix-community/nixos-anywhere/blob/abb0d72...
This is only true if you use the disko-install tool, which is a horrible footgun[^1]. The safest approach is to just use the default disko command, then nixos-install.
[^1]: https://github.com/nix-community/disko/issues/947