There is nothing stopping a Wayland compositor from exposing an interface that would allow for a choice of "window manager". In fact, wlroots could almost count as such a compositor - it implements the bulk of a compositor, but none of the bits of a window manager. Of course, Plasma and Gnome also allow window managers to be integrated as plugins, but I presume that is not what you want.

It is not like the X window manager idea is impeccable either: To name one thing, picom or other compositors could display much nicer and context aware animations if only the window manager interface was not like it is.

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.

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.