Honestly the only build system/package manager that could even compare to portage in versatility is spack. It's aimed at HPC so it's not really oriented towards

Honestly the only build system/package manager that could even compare to portage in versatility is spack. It's aimed at HPC so it's not really oriented towards desktop or even normal server packaging but it gives you what is basically the use-flags and slots systems. And bonus points for striving to get nix style reproducibility.