Yeah I saw your config when lurking around GH for some haumea code the other day.
Really neet and well structured.
My use-case though is different, I’m having a range of different host (w/ different hardware platforms) + a range of users to deploy.
My config is still growing, but from an abstract perspective it looks like nixosModules lets me build templates and have all forms of ‘interconnected’/ re-used modules. My metaphor is that my nixosModules are lego blocks, my templates are the building instructions, a machine configuration the final ‘set’ 
In that scenario it feels like haumea would be a great fit within a more complex nixosModule to make those easier to write. I also really like the visibility feature.
Plz correct me if I’m wrong (I also havent fully understood the possibilities transformers add), but I feel a limitation of haumea is that the nix expression one is writing needs to translatable into a tree-like filesystem hierarchy?