Guix uses GRUB like most distros (but is scripted a bit to allow booting earlier generations of the system). The init system is Guile (Shepherd).

Fewer packages, yes, but the packages are by far the most common ones. It's easy to add packages for yourself, if needed. Nonguix channel and others for stuff upstream won't accept.

I believe Guix is innovating on a number of things in relation to Nix these days. So I've heard. I don't know much about Nix, honestly.

I heartily agree with your last paragraph in the case of Guix.