> something that can track whether things need building from source, or can be brought in as binary from an apt or rpm, or build those packages as intermediate stages. I kinda thought that's what Guix was going to do.

I mean, that sortof what nix (and I assume guix) is though, right? As long as your ok with the binary cache for a replacement for apt/rpm (with the caveat that sometimes pkgs can be built on .deb or .rpm)