Using Scheme instead of a purely functional language like Nix as the Nix/Guix expression language is a bold choice I am not sure I agree with.

PulseAudio != systemd

I have been a very happy NixOS user for 2 1/2 years now! (https://github.com/axelf4/nixos-config)

Not GP, but NixOS makes it easy to make new distros from a NixOS configuration, with ISOs and everything. See for example SnowflakeOS. This is IMO healthier than all the Arch clones etc., since all differences from the base OS are easy to spot, there is little duplication of effort, and it possible to revert to a base NixOS from e.g. SnowflakeOS if a user so would desire.